ML 2.1 QA: Remove deprecated methods for ML

Remove deprecated methods for ML.

This task removed the following (deprecated) public APIs in org.apache.spark.ml:

• classification.RandomForestClassificationModel.numTrees (This now refers to the Param called "numTrees")
• feature.ChiSqSelectorModel.setLabelCol
• regression.LinearRegressionSummary.model
• regression.RandomForestRegressionModel.numTrees (This now refers to the Param called "numTrees")
• PipelineStage.validateParams
• Evaluator.validateParams

This task made the following changes to match existing patterns for Params:

• These methods were made final:
  • classification.RandomForestClassificationModel.getNumTrees
  • regression.RandomForestRegressionModel.getNumTrees
• These methods return the concrete class type, rather than an arbitrary trait. This only affected Java compatibility, not Scala.
  • classification.RandomForestClassificationModel.setFeatureSubsetStrategy
  • regression.RandomForestRegressionModel.setFeatureSubsetStrategy