[SPARK-11559][MLLIB] Make `runs` no effect in mllib.KMeans

## What changes were proposed in this pull request?
We deprecated  ```runs``` of mllib.KMeans in Spark 1.6 (SPARK-11358). In 2.0, we will make it no effect (with warning messages). We did not remove ```setRuns/getRuns``` for better binary compatibility.
This PR change `runs` which are appeared at the public API. Usage inside of ```KMeans.runAlgorithm()``` will be resolved at #10806.

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

cc jkbradley

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #12608 from yanboliang/spark-11559.