(fix): remove filterwarning for observed=False

[ilan-gold]

• Closes Future warning in preprocessing._highly_variable_genes.py:226 #2967
• Tests included or not required because: Just removing a warning from the filter

• Release notes not necessary because: just a dev change

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.74%. Comparing base (def4f23) to head (36a036a).

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3639   +/-   ##
=======================================
  Coverage   76.74%   76.74%           
=======================================
  Files         113      113           
  Lines       12709    12709           
=======================================
  Hits         9754     9754           
  Misses       2955     2955           

[ilan-gold]

I would also understand not merging this, but we do not have behavior described by the issue anymore so I would think we could remove this warning.