Eval dataset should not be balanced

[AdamChit]

Related issues
validationPrepare was being called on the validation datasets which will balance the dataset. This should not be done because the validation set should represent the real distribution.

Describe the proposed solution
remove the call to validationPrepare

[codecov]

Codecov Report

Merging #429 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #429      +/-   ##
==========================================
- Coverage   86.97%   86.96%   -0.02%     
==========================================
  Files         337      337              
  Lines       11082    11082              
  Branches      355      588     +233     
==========================================
- Hits         9639     9637       -2     
- Misses       1443     1445       +2

Impacted Files	Coverage Δ
...op/stages/impl/tuning/OpTrainValidationSplit.scala	100% <ø> (ø)	⬆️
...orce/op/stages/impl/tuning/OpCrossValidation.scala	97.95% <ø> (ø)	⬆️
...es/src/main/scala/com/salesforce/op/OpParams.scala	85.71% <0%> (-4.09%)	⬇️

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[tovbinm]

@leahmcguire @AdamChit didn't we just change it in this PR - #424 ?

[leahmcguire]

Yes @tovbinm I was too quick to make the change :-)

[gerashegalov]

@AdamChit @leahmcguire Can we capture the expected behavior in a test to reduce the risk of merging "too quick"?