Complete migration of transformers out of this package

[ablaom]

This PR removes all the models currently provided by "src/builtins/Transformers.jl" (OneHotEncoding, FillImputer, etc) and adds MLJTransforms.jl to the MLJ Model Registry, which now provides these models.

Some orphaned legacy code is also removed from "test"GaussianProcesses.jl, NaiveBayes.jl).

See #591 for context.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.27%. Comparing base (c35207e) to head (d3d068b).
⚠️ Report is 7 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #592      +/-   ##
==========================================
- Coverage   85.33%   76.27%   -9.07%     
==========================================
  Files           9        8       -1     
  Lines         914      472     -442     
==========================================
- Hits          780      360     -420     
+ Misses        134      112      -22     

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