deprecate legacy resolve mode

[davidism]

Changed the metaclass logic to check if the names were defined on the child class instead of comparing the function objects. _fast_resolve_mode was never getting triggered, which suggested comparing the objects wasn't completely reliable anyway. Inlined the resolve_or_missing function into both methods.

• fixes remove Context._legacy_resolve_mode #1380

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.