Remove libclang pin from clang dependency group

[vyasr]

This PR removes the libclang==20.1.8 pin from the clang dependency group in dependencies.yaml.

Problem

The explicit libclang==20.1.8 pin causes the conda solver to hang when creating the combined RAPIDS devcontainer environment. Specifically, when cuvs is included alongside cudf (which depends on matplotlib<3.11), the solver cannot efficiently resolve the conflict between:

• libclang==20.1.8 → provides libclang13 20.1.8
• matplotlib<3.11 → pulls in qt6-main → requires libclang13>=22.1.0

While the environment is technically solvable (older Qt builds accept libclang13>=20.1.8), the search space created by the 29 builds of libclang 20.1.8 × multiple Qt versions × CUDA packages × rapidsai channel packages causes the solver to hang indefinitely.

Why this is safe to remove

1. clang and clang-tools do not depend on libclang. The clang compiler depends on libclang-cpp (the C++ API), which is a separate package from libclang (the C API wrapper).

2. The Rust bindgen workflow doesn't use the clang dependency group. The rust file key does not include the clang group — bindgen finds libclang.so at runtime from whatever version is available in the environment (provided transitively via Qt/matplotlib's dependency on libclang13).

3. Other RAPIDS repos don't pin libclang. cudf, raft, and rmm only pin clang and clang-tools without libclang and do not experience this issue.

4. Historical context: libclang was originally added to the rust dependency group for bindgen support, then moved into the clang group during a version bump refactor. The rust file key no longer includes the clang group, making this entry vestigial.