Backport PR #4202: docs: Remove duplicated 'layer' parameter from PCA documentation

[ilan-gold]

• Closes #
• Tests included or not required because:

• Release notes not necessary because:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.62%. Comparing base (5aa9751) to head (74f8545).
⚠️ Report is 1 commits behind head on 1.12.x.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           1.12.x    #4216   +/-   ##
=======================================
  Coverage   78.62%   78.62%           
=======================================
  Files         121      121           
  Lines       12707    12707           
=======================================
  Hits         9991     9991           
  Misses       2716     2716           

Flag	Coverage Δ
hatch-test.low-vers	77.68% <ø> (ø)
hatch-test.pre	78.51% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/scanpy/preprocessing/_pca/__init__.py	90.65% <ø> (ø)