chore: Clean remove_prefix since python3.9 is now the minimum Python

[MarcoGorelli]

• Python3.9 has removeprefix, so the _remove_prefix utily isn't needed
• Generator[Any] instead of Generator[Any, None, None] (https://peps.python.org/pep-0696/)
• walrus operator in some bits where I think it's tasty - it's already used in quite a few places, I guess Polars has already decided to embrace it 😄

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.78%. Comparing base (f7de80c) to head (7e5b615).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
py-polars/polars/dataframe/frame.py	0.00%	0 Missing and 1 partial ⚠️
py-polars/polars/lazyframe/in_process.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19070      +/-   ##
==========================================
- Coverage   79.80%   79.78%   -0.02%     
==========================================
  Files        1531     1531              
  Lines      208563   208556       -7     
  Branches     2913     2913              
==========================================
- Hits       166437   166405      -32     
- Misses      41575    41600      +25     
  Partials      551      551              

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[alexander-beedie]

🙃👍