[SPARK-14300][DOCS][MLLIB] Scala MLlib examples code merge and clean up

## What changes were proposed in this pull request?

https://issues.apache.org/jira/browse/SPARK-14300

Duplicated code found in scala/examples/mllib, below all deleted in this PR:

- DenseGaussianMixture.scala
- StreamingLinearRegression.scala

## delete reasons:

#### delete: mllib/DenseGaussianMixture.scala

- duplicate of mllib/GaussianMixtureExample

#### delete: mllib/StreamingLinearRegression.scala

- duplicate of mllib/StreamingLinearRegressionExample

When merging and cleaning those code, be sure not disturb the previous example on and off blocks.

## How was this patch tested?

Test with `SKIP_API=1 jekyll` manually to make sure that works well.

Author: Xin Ren <iamshrek@126.com>

Closes #12195 from keypointt/SPARK-14300.