[SPARK-11560][MLLIB] Optimize KMeans implementation / remove 'runs'

## What changes were proposed in this pull request?

This is a revival of #14948 and related to #14937. This removes the 'runs' parameter, which has already been disabled, from the K-means implementation and further deprecates API methods that involve it.

This also happens to resolve the issue that K-means should not return duplicate centers, meaning that it may return less than k centroids if not enough data is available.

## How was this patch tested?

Existing tests

Author: Sean Owen <sowen@cloudera.com>

Closes #15342 from srowen/SPARK-11560.