🔥 Eliminate all logic related to using_key in curators

[falexwolf]

Is part of a sequence of PRs that refactors the curators:

• ♻️ Refactor organism handling in curators #2415
• 🚚 Bulk-rename old-style curators to CatCurator #2413
• 🎨 Self-contained definition of CellxGene schema #2412
• 🚚 Move PertCurator from wetlab here and add CellxGene Curator test #2408
• 🚚 Move CellxGene Curator from cellxgene-lamin here #2403
• ✨ Schema-based curators: DataFrameCurator #2388

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Removing all logic related to using_key in curators is desirable because:

1. it gives rise to a leaner definition of a "valid category" that no longer involves the state of another database, but is restricted to the state of the current database & well-defined public ontology versions
2. it eliminates much code that makes refactoring easier

This is possible because PR 2412 eliminated the need for passing using_key for validating against the CellxGene schema, whose non-ontology based control values were its only motivation in the first place.

using_key was never a user-facing argument and hence its removal doesn't break backward compat.

[codecov]

Codecov Report

Attention: Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.48%. Comparing base (d503387) to head (da2e7aa).
Report is 33 commits behind head on main.

Files with missing lines	Patch %	Lines
lamindb/curators/__init__.py	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2414      +/-   ##
==========================================
- Coverage   91.71%   91.48%   -0.23%     
==========================================
  Files          62       62              
  Lines        9138     9610     +472     
==========================================
+ Hits         8381     8792     +411     
- Misses        757      818      +61     

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