Use Woodwork public get_subset_schema instead of private method

[angela97lin]

Use Woodwork public get_subset_schema instead of private method, closes #3168

[codecov]

Codecov Report

Merging #3325 (afc1b76) into main (562d202) will not change coverage.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main   #3325   +/-   ##
=====================================
  Coverage   99.8%   99.8%           
=====================================
  Files        324     324           
  Lines      31764   31764           
=====================================
  Hits       31674   31674           
  Misses        90      90           

Impacted Files	Coverage Δ
...transformers/feature_selection/feature_selector.py	100.0% <100.0%> (ø)
...onents/transformers/imputers/per_column_imputer.py	100.0% <100.0%> (ø)
...components/transformers/imputers/simple_imputer.py	100.0% <100.0%> (ø)
...ponents/transformers/preprocessing/featuretools.py	100.0% <100.0%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 562d202...afc1b76. Read the comment docs.

[thehomebrewnerd]

LGTM!