Remove custom MRO implementation of Pydantic models

[Viicos]

This is no longer necessary since #10666 was introduced.

Fixes #11183.
Fixes #11042.
Closes #11037.

Change Summary

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• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable
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[Viicos]

I believe this should be backported, as it fixes #11042 which was a regression.

[sydney-runkle]

Generally, I approve of this approach, as discussed in #10666. I think that the less we can manipulate the MRO, the better, as it introduces a pretty complex internal pattern that's hard to maintain.

That being said, I think we should get buy in from @kc0506 and @dmontagu, as they were pretty involved in the addition of this custom MRO implementation.

Happy to do another patch 2.10 release with this fix, I've added the corresponding label.

[codspeed]

CodSpeed Performance Report

Merging #11184 will not alter performance

_{Comparing revert-mro (6adc9c3) with main (fac2bfd)}

Summary

✅ 46 untouched benchmarks

[github-actions]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
pydantic/_internal
_model_construction.py
Project Total

This report was generated by python-coverage-comment-action

[sydney-runkle]

Reviewed again, given that this is basically just a reversion, I think we can go ahead and merge.