docs(progress): remove reverted IterationSpeedColumn

[TomJGooding]

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Remove IterationSpeedColumn from the progress documentation, as this addition was reverted not long after being merged (see https://github.com/Textualize/rich/pull/3332#issuecomment-2383977334).

Closes #3739.

Type of changes

• [ ] Bug fix
• [ ] New feature
• [x] Documentation / docstrings
• [ ] Tests
• [ ] Other

Checklist

• [ ] I've run the latest black with default args on new code.
• [ ] I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• [ ] I've added tests for new code.
• [x] I accept that @willmcgugan may be pedantic in the code review.

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.98%. Comparing base (225cb07) to head (669b500). Report is 38 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3742      +/-   ##
==========================================
- Coverage   98.24%   97.98%   -0.27%     
==========================================
  Files          74       74              
  Lines        8105     8142      +37     
==========================================
+ Hits         7963     7978      +15     
- Misses        142      164      +22     

Flag	Coverage Δ
unittests	97.98% <ø> (-0.27%)	:arrow_down:

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[PythonFZ]

Would it be possible to reintroduce this feature?

[willmcgugan]

Would it be possible to reintroduce this feature?

Just needs someone to write the PR.