Simplify var_groups

[flying-sheep]

• Closes #
• Tests included or not required because:

• Release notes not necessary because:

[codecov]

Codecov Report

Attention: Patch coverage is 80.85106% with 9 lines in your changes missing coverage. Please review.

Project coverage is 75.40%. Comparing base (cce4bd6) to head (d3f4dfc).

Files with missing lines	Patch %	Lines
src/scanpy/plotting/_baseplot_class.py	80.43%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3462      +/-   ##
==========================================
- Coverage   75.44%   75.40%   -0.05%     
==========================================
  Files         113      113              
  Lines       13250    13259       +9     
==========================================
+ Hits         9997     9998       +1     
- Misses       3253     3261       +8     

Files with missing lines	Coverage Δ
src/scanpy/neighbors/_connectivity.py	85.96% <100.00%> (ø)
src/scanpy/plotting/_baseplot_class.py	89.56% <80.43%> (-1.99%)	⬇️