[DO NOT MERGE] Remove existing nullable type handlings across automl

[tamargrey]

Pull Request Description

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---

After creating the pull request: in order to pass the release_notes_updated check you will need to update the "Future Release" section of docs/source/release_notes.rst to include this pull request by adding :pr:123.

[machineFL]

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[codecov]

Codecov Report

Merging #4055 (274f114) into use-nullable-type-handling (6a10435) will decrease coverage by 39.8%.
The diff coverage is 52.9%.

@@                      Coverage Diff                      @@
##           use-nullable-type-handling   #4055      +/-   ##
=============================================================
- Coverage                        99.7%   59.9%   -39.8%     
=============================================================
  Files                             349     350       +1     
  Lines                           37579   37595      +16     
=============================================================
- Hits                            37461   22495   -14966     
- Misses                            118   15100   +14982     

Impacted Files	Coverage Δ
...omponents/estimators/regressors/arima_regressor.py	79.9% <ø> (-20.1%)	⬇️
...onents/estimators/regressors/catboost_regressor.py	97.8% <ø> (-2.2%)	⬇️
...tors/regressors/exponential_smoothing_regressor.py	78.5% <ø> (-21.5%)	⬇️
...nents/transformers/imputers/time_series_imputer.py	80.6% <ø> (-19.4%)	⬇️
...es/components/transformers/samplers/oversampler.py	95.9% <ø> (-4.1%)	⬇️
evalml/pipelines/utils.py	84.1% <ø> (-15.5%)	⬇️
...valml/tests/automl_tests/test_default_algorithm.py	100.0% <ø> (ø)
...alml/tests/component_tests/test_arima_regressor.py	0.0% <0.0%> (-100.0%)	⬇️
...nent_tests/test_exponential_smoothing_regressor.py	0.0% <0.0%> (-100.0%)	⬇️
evalml/tests/component_tests/test_imputer.py	0.0% <0.0%> (-100.0%)	⬇️
... and 17 more

... and 186 files with indirect coverage changes

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[tamargrey]

closing in favor of #4085