[SPARK-14301][EXAMPLES] Java examples code merge and clean up.

## What changes were proposed in this pull request?

This fix tries to remove duplicate Java code in examples/mllib and examples/ml. The following changes have been made:

```
deleted: ml/JavaCrossValidatorExample.java (duplicate of JavaModelSelectionViaCrossValidationExample.java)
deleted: ml/JavaTrainValidationSplitExample.java (duplicated of JavaModelSelectionViaTrainValidationSplitExample.java)
deleted: mllib/JavaFPGrowthExample.java (duplicate of JavaSimpleFPGrowth.java)
deleted: mllib/JavaLDAExample.java (duplicate of JavaLatentDirichletAllocationExample.java)
deleted: mllib/JavaKMeans.java (merged with JavaKMeansExample.java)
deleted: mllib/JavaLR.java (duplicate of JavaLinearRegressionWithSGDExample.java)
updated: mllib/JavaKMeansExample.java (merged with mllib/JavaKMeans.java)
```

## How was this patch tested?
Existing tests passed.

Author: Yong Tang <yong.tang.github@outlook.com>

Closes #12143 from yongtang/SPARK-14301.