Dont allow knn as boolean impute strategy

[tamargrey]

closes #4057

Removes "knn" from boolean_impute_strategy in the Imputer component and stops accounting for boolean data in the KNNImputer component.

[codecov]

Codecov Report

Merging #4058 (5ed40c3) into main (f509df4) will decrease coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #4058     +/-   ##
=======================================
- Coverage   99.7%   99.7%   -0.0%     
=======================================
  Files        349     349             
  Lines      37509   37476     -33     
=======================================
- Hits       37391   37358     -33     
  Misses       118     118             

Impacted Files	Coverage Δ
evalml/tests/pipeline_tests/test_pipelines.py	99.9% <ø> (ø)
...elines/components/transformers/imputers/imputer.py	100.0% <100.0%> (ø)
...es/components/transformers/imputers/knn_imputer.py	100.0% <100.0%> (ø)
evalml/tests/component_tests/test_imputer.py	100.0% <100.0%> (ø)
evalml/tests/component_tests/test_knn_imputer.py	100.0% <100.0%> (ø)

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[jeremyliweishih]

LGTM