Remove unused point cloud color uniform variable

[iche033]

🦟 Bug fix

Fixes #719

We're not longer using the color uniform in the shaders so removing it to prevent a warning from popping up in the ogre2.log on macOS

Checklist

• Signed all commits for DCO
• 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)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #721 (aed9158) into gz-rendering7 (ff5d75e) will not change coverage.
The diff coverage is n/a.

@@              Coverage Diff               @@
##           gz-rendering7     #721   +/-   ##
==============================================
  Coverage          74.14%   74.14%           
==============================================
  Files                164      164           
  Lines              14351    14351           
==============================================
  Hits               10641    10641           
  Misses              3710     3710           

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