[SPARK-11358][MLLIB] deprecate runs in k-means

This PR deprecates `runs` in k-means. `runs` introduces extra complexity and overhead in MLlib's k-means implementation. I haven't seen much usage with `runs` not equal to `1`. We don't have a unit test for it either. We can deprecate this method in 1.6, and void it in 1.7. It helps us simplify the implementation.

cc: srowen

Author: Xiangrui Meng <meng@databricks.com>

Closes #9322 from mengxr/SPARK-11358.