fix single lead dim not dropped in remove_bias

[aaronspring]

Description

CLIMPRED_ENSEMBLE_DIMS shouldnt be dropped

Closes #771

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Tests added for pytest, if necessary.

Checklist (while developing)

• CHANGELOG is updated with reference to this PR.

[github-actions]

Checkout your PR on . Open the console and pip install -e . before using the notebooks to use climpred from this PR.

[codecov]

Codecov Report

Merging #773 (f102941) into main (dafd1f3) will decrease coverage by 1.07%.
The diff coverage is 90.13%.

@@            Coverage Diff             @@
##             main     #773      +/-   ##
==========================================
- Coverage   93.80%   92.73%   -1.08%     
==========================================
  Files          59       59              
  Lines        6235     6209      -26     
==========================================
- Hits         5849     5758      -91     
- Misses        386      451      +65     

Impacted Files	Coverage Δ
climpred/horizon.py	96.55% <ø> (ø)
climpred/options.py	81.48% <ø> (ø)
climpred/tests/test_hindcast_prediction.py	100.00% <ø> (ø)
climpred/tests/test_logging.py	100.00% <ø> (ø)
climpred/bootstrap.py	80.52% <73.07%> (-10.36%)	⬇️
climpred/alignment.py	91.13% <73.68%> (-5.74%)	⬇️
climpred/checks.py	95.79% <83.33%> (+0.03%)	⬆️
climpred/tests/test_HindcastEnsemble_class.py	96.51% <85.71%> (-3.49%)	⬇️
climpred/utils.py	88.69% <86.66%> (-1.70%)	⬇️
climpred/classes.py	93.36% <87.35%> (-0.15%)	⬇️
... and 37 more

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