docs: drop the duplicated UPDATING.md sections left by the 0.3.0 rewrite

[helix-nine]

UPDATING.md has been carrying three orphaned chunks of an older revision of itself, including a second, divergent copy of the entire Applying the bump section. A packager following the file could read either one.

What happened

The 0.3.0 doc rewrite (2740ee8) inserted its new content above the old text instead of replacing it. Three pieces of the previous revision were left behind:

• the tail of the intro paragraph, landing mid-sentence right after "The tag has two parts"
• the tail of the old "Determining the upstream version", as a stray indented block under step 4
• the whole old 3-step "Applying the bump", as a second section with the same heading

A later rename pass then swapped lundog/simplex-chat → lundog/simplex-websocket-bridge inside those orphans, so the dead copy reads as current rather than obviously stale. That is most likely why it survived this long.

What this changes

Drops all three orphans. The surviving "Applying the bump" is a strict superset of the dead one — it adds the image-must-be-rebuilt-first step and the file-exchange defaults re-check, neither of which the old copy had. The one fact the dead copy stated on its own, where the pin lives in manifest/index.ts, is already given immediately above the section.

Corrects the -N revision guidance. Step 2 told the packager unconditionally to "keep the explicit -N revision", which the repo cannot follow: per the file's own explainer an unsuffixed tag is revision 0, so a first build of a SimpleX version has no suffixed form to pin. The manifest's bare 6.5.6 pin was following that, not deviating from it. Both the explainer and step 2 now say to pin the explicit revision where one exists, which keeps the original intent — don't let the tag shift underneath you.

Verification

Docs only; no source, manifest, or version change, so the build is untouched.

• Each ## heading now appears exactly once, and the file reads top to bottom with no mid-sentence breaks.
• Re-checked the two factual claims this keeps, against Docker Hub rather than trusting them: the old lundog/simplex-chat repo is indeed frozen at 6.5.5-1 with no newer builds, and 6.5.5 / 6.5.5-2 are genuinely the same image (both sha256:f46cca57449e…).
• Line wrapping matches the file's existing hand-wrapped width; no formatter governs root markdown here (the prettier script is scoped to startos/).

Note on #12

This is independent of #12 and does not conflict with it — that PR does not touch UPDATING.md.

One follow-up is deliberately not included here: step 4 says to bump the package version without stating how to choose it, which is correct under the current wrapper-versioning scheme. If #12's renumbering half lands, this step should gain the rule that the package version tracks the bundled SimpleX version. Left out on purpose, since that half was explicitly offered as revertible.

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