fix: remove _saved_python when purging venv

[BVMiko]

Pull Request Checklist

• A news fragment is added in news/ describing what is new.
• Test cases added for changed code.

Describe what you have changed in this PR.

The command pdm venv remove in-project --yes will remove the .pdm-python file if target of the python path is in the removed venv folder, but the pdm venv purge command does not. This updates the pdm venv purge command to match the behavior of the former.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 85.67%. Comparing base (7cb963d) to head (2bb3d73).
Report is 93 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pdm/cli/commands/venv/purge.py	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3137      +/-   ##
==========================================
- Coverage   85.68%   85.67%   -0.02%     
==========================================
  Files         109      109              
  Lines       11037    11041       +4     
  Branches     2411     2412       +1     
==========================================
+ Hits         9457     9459       +2     
- Misses       1090     1091       +1     
- Partials      490      491       +1     

Flag	Coverage Δ
unittests	85.47% <50.00%> (-0.02%)	⬇️

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