Remove unregistered rtx_raytracing_fractionalCutoutOpacity carb setting

[lanafi00]

Description

Isaac Sim 5.1 no longer registers rtx_raytracing_fractionalCutoutOpacity in carb settings, so Isaac Lab's _apply_render_settings_from_cfg() was raising a ValueError because this key is not found.

I don't know how rtx_raytracing_fractionalCutoutOpacity was renamed or restructured so I dropped the stale key entirely.
This is a raytraced-cutout-opacity rendering toggle, not physics-affecting, so nothing behavioral is lost beyond that visual effect.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have run the pre-commit checks with <FULL_PATH_TO_ISAACLAB>/isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there

[SissiFeng]

Request changes: please rebase and reproduce this issue against current main before deciding to remove the setting.

Please complete the following:

1. Run an environment-construction smoke test using the exact versions targeted by current main:

   • start Isaac Sim/Isaac Lab;
   • construct a Matterix environment with one environment instance;
   • call reset() and at least one step();
   • close the environment cleanly;
   • report any exception or new warning.

2. Record the actual runtime versions of Isaac Lab, Isaac Sim, Python, CUDA, and the renderer. The existing queued CI job does not provide this evidence.

3. Test the documented replacement setting:
   /rtx/pathtracing/fractionalCutoutOpacity

   Please determine whether the original translucency intent can be preserved using the current RenderCfg API, for example with enable_translucency=True plus the current carb setting path.

4. Add a configuration/runtime test that verifies:

   • the obsolete path is not used;
   • the intended current setting is present;
   • the value read back from carb settings after environment construction is correct.

5. Provide before/after renders using the same scene, camera, lighting, seed, and renderer. The scene should contain representative transparent or cutout materials such as glassware or liquid. Please attach both images and check for opacity, missing cutout detail, and translucency artifacts.

6. Update the in-code comment to describe the current Isaac Lab 3.0 behavior rather than the obsolete 2.3 method and Isaac Sim 5.1 failure path.

If the exception cannot be reproduced on the current stack, this PR should be closed or re-scoped as cleanup. If fractional cutout opacity is still required, the PR should migrate to the supported setting instead of deleting the visual behavior without regression evidence.

[lanafi00]

I could not reproduce this error on the current stack, so I am re-scoping as cleanup.

Reproduction, old stack: confirmed on isaaclab==2.3.0 / isaacsim==5.1.0.0. Constructing SimulationContext with rtx_raytracing_fractionalCutoutOpacity in carb_settings raises:
ValueError: '/rtx/raytracing/fractionalCutoutOpacity' in RenderCfg.general_parameters does not map to a carb setting.

Reproduction, current main: does not reproduce on isaaclab==3.0.0b2.post1 / isaacsim==6.0.1.0. SimulationContext's render-settings application was rewritten between these versions, and isaaclab==3.0.0b2.post1's version no longer checks for the existence of carb settings.

Correction on the replacement-setting question: Correcting my earlier PR, this setting is lazily registered, not retired. rtx_raytracing_fractionalCutoutOpacity is raytracing-namespaced, and only gets registered once the renderer actually switches into that render mode. SimulationContext.__init__(), which is where the base env's carb_settings get applied, always runs before any switch to raytracing render mode.

. Confirmed identically on both Isaac Sim versions: searching the /rtx tree right after launch found nothing on either 5.1.0 or 6.0.1.0, but once I forced actual PathTracing render mode and rendered a real frame, the setting appeared on both with real values (/rtx/raytracing/fractionalCutoutOpacity=False, /rtx/pathtracing/fractionalCutoutOpacity=True by default).

Translucency preservation: rtx_translucency_enabled is a separate setting, unaffected by any of this, and confirmed still applying correctly.

Config/runtime test (source/matterix/test/test_carb_settings_translucency.py): the base config never reintroduces the dead key, and rtx_translucency_enabled is present and reads back True after a real environment construction. Verified directly on isaaclab==3.0.0b2.post1/isaacsim==6.0.1.0.

Before/after renders: captured the beaker-lift scene (default render mode, not path tracing) with identical camera/lighting/seed, dead key present vs. removed. Byte-identical MD5.

In-code comment: rewritten to describe the actual mechanism rather than the incorrect "retired" claim.