Remove CategoricalEncoder base class and get_feature_names function

[dsherry]

Problem
The get_feature_names accessor doesn't work, now that our one-hot encoder is more than a wrapper around the sklearn one-hot encoder, and we aren't using the component_obj code path for it.

Repro

import pandas as pd
import numpy as np
import evalml
X, y = evalml.demos.load_breast_cancer()
ohe = evalml.pipelines.components.OneHotEncoder()
ohe.fit(X, y=y)
ohe.get_feature_names()

Results in

AttributeError: 'NoneType' object has no attribute 'feature_names'

Proposal
Let's delete get_feature_names. We have a different API in place for accessing the list of the names of the features provided to each component in a pipeline.

At that point, CategoricalEncoder is just an empty class only referenced by one other piece of code. So we should remove it.

[codecov]

Codecov Report

Merging #1179 into main will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1179   +/-   ##
=======================================
  Coverage   99.91%   99.92%           
=======================================
  Files         197      196    -1     
  Lines       11705    11700    -5     
=======================================
- Hits        11695    11691    -4     
+ Misses         10        9    -1     

Impacted Files	Coverage Δ
evalml/pipelines/__init__.py	100.00% <ø> (ø)
evalml/pipelines/components/__init__.py	100.00% <ø> (ø)
...lines/components/transformers/encoders/__init__.py	100.00% <ø> (ø)
...alml/pipelines/components/transformers/__init__.py	100.00% <100.00%> (ø)
...components/transformers/encoders/onehot_encoder.py	100.00% <100.00%> (ø)

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[angela97lin]

Thanks for this cleanup! LGTM 👍

[dsherry]

Note I filed a separate issue #1183 to track adding this attribute back in, with proper support.