Remove obsolete RQt source installation instructions. (backport #4796)

[mergify]

These are no longer needed, since the RQt source installation instructions are part of the "standard" development setup.

Usually I would consider making links for these, but given how outdated these are, I'd rather just remove them completely.

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This is an automatic backport of pull request #4796 done by Mergify.

[mergify]

Cherry-pick of 14bdbb5 has failed:

On branch mergify/bp/iron/pr-4796
Your branch is up to date with 'origin/iron'.

You are currently cherry-picking commit 14bdbb51.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   source/Concepts/Intermediate/About-RQt.rst
	modified:   source/How-To-Guides.rst
	modified:   source/How-To-Guides/Installation-Troubleshooting.rst
	deleted:    source/How-To-Guides/RQt-Source-Install-MacOS.rst
	deleted:    source/How-To-Guides/RQt-Source-Install-Windows10.rst
	deleted:    source/How-To-Guides/RQt-Source-Install.rst

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by them: source/How-To-Guides/RQt-Port-Plugin-Windows.rst

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[github-actions]

HTML artifacts: https://github.com/ros2/ros2_documentation/actions/runs/11148077724/artifacts/2007031626.

To view the resulting site:

1. Click on the above link to download the artifacts archive
2. Extract it
3. Open html-artifacts-4798/index.html in your favorite browser