Revert "Conditionally include the imputer in pipelines (#3657)"

[chukarsten]

This reverts commit ceebe08.

Pull Request Description

(replace this text with your 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.

[codecov]

Codecov Report

Merging #3672 (05698a1) into main (85472b6) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #3672     +/-   ##
=======================================
- Coverage   99.7%   99.7%   -0.0%     
=======================================
  Files        337     337             
  Lines      34021   34018      -3     
=======================================
- Hits       33890   33887      -3     
  Misses       131     131             

Impacted Files	Coverage Δ
evalml/pipelines/classification_pipeline.py	100.0% <ø> (ø)
...valml/tests/automl_tests/test_default_algorithm.py	100.0% <ø> (ø)
evalml/pipelines/utils.py	99.5% <100.0%> (ø)
evalml/tests/automl_tests/test_automl.py	99.5% <100.0%> (+0.1%)	⬆️
...ts/automl_tests/test_automl_iterative_algorithm.py	100.0% <100.0%> (ø)
evalml/tests/pipeline_tests/test_pipeline_utils.py	99.8% <100.0%> (ø)

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