refactor: remove _CmapCombo thin subclass

[tlambert03]

pyapp-kit/superqt#333 includes the fixes we had in our cmap combobox thin subclass here.

and also, our popup geometry overrides were flaky. This removes both and goes back to a stock combo

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.79%. Comparing base (a37e777) to head (9b7673e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #262      +/-   ##
==========================================
+ Coverage   86.76%   86.79%   +0.03%     
==========================================
  Files          50       50              
  Lines        5742     5725      -17     
==========================================
- Hits         4982     4969      -13     
+ Misses        760      756       -4     

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