Refactor and remove SamplerBase class

[angela97lin]

Closes #2607

Removes SamplerBase class, in favor of BaseSampler. BalancedClassificationSampler was a subclass of SamplerBase and only used as a component object of the Undersampler class, so simplified by removing the BalancedClassificationSampler class and moving the implementation to Undersampler.

[codecov]

Codecov Report

Merging #2775 (d1ce08f) into main (2719871) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #2775     +/-   ##
=======================================
- Coverage   99.8%   99.8%   -0.0%     
=======================================
  Files        301     298      -3     
  Lines      27904   27595    -309     
=======================================
- Hits       27841   27527    -314     
- Misses        63      68      +5     

Impacted Files	Coverage Δ
evalml/preprocessing/data_splitters/__init__.py	100.0% <ø> (ø)
...s/components/transformers/samplers/undersampler.py	100.0% <100.0%> (ø)
evalml/tests/component_tests/test_undersampler.py	100.0% <100.0%> (ø)
...s/components/transformers/samplers/base_sampler.py	89.2% <0.0%> (-10.8%)	⬇️

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[chukarsten]

Just a few hypernits and an ask for an unnecessary comment cleanup! But awesome, this looks good.

[chukarsten]

Is it worth getting ahead of the game and adding "raises" for these newish exceptions to the docstring?

[angela97lin]

Love it! Unfortunately, the linting packages aren't smart enough to tell that the class docstring should be connected to the init docstring but going to add this 😁