Remove Tf2 time tutorials for Python (backport #4384)

[mergify]

There are currently 4 tutorials having to do with time for tf2:

C++:

• https://github.com/ros2/ros2_documentation/blob/5c6c6140d3e111b1ba8a721c937bbd1792eef9ad/source/Tutorials/Intermediate/Tf2/Learning-About-Tf2-And-Time-Cpp.rst
• https://github.com/ros2/ros2_documentation/blob/5c6c6140d3e111b1ba8a721c937bbd1792eef9ad/source/Tutorials/Intermediate/Tf2/Time-Travel-With-Tf2-Cpp.rst

Python:

• https://github.com/ros2/ros2_documentation/blob/5c6c6140d3e111b1ba8a721c937bbd1792eef9ad/source/Tutorials/Intermediate/Tf2/Learning-About-Tf2-And-Time-Py.rst
• https://github.com/ros2/ros2_documentation/blob/5c6c6140d3e111b1ba8a721c937bbd1792eef9ad/source/Tutorials/Intermediate/Tf2/Time-Travel-With-Tf2-Py.rst

The C++ ones work fine, and have worked fine since approximately forever. This PR does some small cleanups in those tutorials, but otherwise leaves them alone.

The Python ones are a different story. Due to some bugs in tf2_py itself, those tutorials have never worked in ROS 2. The correct solution here would be to fix up tf2_py to make them work, but that hasn't yet happened. Thus, I think it is better to remove those tutorials than to continue to provide them in a non-working state. So this PR does exactly that.

It also has some other random small fixups throughout the tf2 tutorials.

Edit: this also "fixes" #3960 and #3080 and #2935 , by removing the offending tutorial completely.

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This is an automatic backport of pull request #4384 done by Mergify.

[mergify]

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Changes to be committed:
	modified:   source/Concepts/Intermediate/About-Tf2.rst
	modified:   source/Tutorials/Intermediate/Tf2/Learning-About-Tf2-And-Time-Cpp.rst
	modified:   source/Tutorials/Intermediate/Tf2/Tf2-Main.rst
	modified:   source/Tutorials/Intermediate/Tf2/Writing-A-Tf2-Static-Broadcaster-Cpp.rst
	modified:   source/Tutorials/Intermediate/Tf2/Writing-A-Tf2-Static-Broadcaster-Py.rst

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   source/Tutorials/Intermediate/Tf2/Adding-A-Frame-Cpp.rst
	both modified:   source/Tutorials/Intermediate/Tf2/Adding-A-Frame-Py.rst
	deleted by them: source/Tutorials/Intermediate/Tf2/Learning-About-Tf2-And-Time-Py.rst
	deleted by them: source/Tutorials/Intermediate/Tf2/Time-Travel-With-Tf2-Py.rst
	both modified:   source/Tutorials/Intermediate/Tf2/Writing-A-Tf2-Broadcaster-Cpp.rst
	both modified:   source/Tutorials/Intermediate/Tf2/Writing-A-Tf2-Broadcaster-Py.rst
	both modified:   source/Tutorials/Intermediate/Tf2/Writing-A-Tf2-Listener-Cpp.rst
	both modified:   source/Tutorials/Intermediate/Tf2/Writing-A-Tf2-Listener-Py.rst

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