package.xml: remove mujoco for now

[scpeters]

🦟 Bug fix

Fixes rosdep resolution for colcon CI

Summary

A recent colcon CI build for gazebosim/gz-math#780 (comment) failed during rosdep resolution:

• https://build.osrfoundation.org/job/ci__colcon_gpu_any-manual_ubuntu_noble_amd64/27/

Traceback (most recent call last):
  File "/tmp/ros_buildfarm/ros_buildfarm/scripts/ci/generate_install_lists.py", line 278, in resolve_names
# END SUBSECTION
    resolved_names = resolve_for_os(
                     ^^^^^^^^^^^^^^^
...
KeyError: 'mujoco'
...
RuntimeError: Could not resolve the rosdep key 'mujoco'

This comments out the mujoco dependency from package.xml until it has a rosdep key.

testing again with this branch:

• https://build.osrfoundation.org/job/ci__colcon_gpu_any-manual_ubuntu_noble_amd64/28/

Checklist

• Signed all commits for DCO
• Added a screen capture or video to the PR description that demonstrates the fix (as needed)
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• Updated Bazel files (if adding new files). Created an issue otherwise.
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• Was GenAI used to generate this PR? If so, make sure to add "Generated-by" to your commits. (See this policy for more info.)

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[scpeters]

this should not be backported