feat: remove legacy pdm-pep517 backend

[frostming]

Signed-off-by: Frost Ming [email protected]

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.

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.01% ⚠️

Comparison is base (c815118) 84.29% compared to head (d9bb2f0) 84.28%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2167      +/-   ##
==========================================
- Coverage   84.29%   84.28%   -0.01%     
==========================================
  Files         100      100              
  Lines        9781     9777       -4     
  Branches     2160     2159       -1     
==========================================
- Hits         8245     8241       -4     
  Misses       1062     1062              
  Partials      474      474              

Flag	Coverage Δ
unittests	84.06% <ø> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
src/pdm/models/backends.py	83.14% <ø> (-0.73%)	⬇️

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