[MINOR][ML][PYSPARK] Cleanup test cases of clustering.py

Test cases should be removed from annotation of ```setXXX``` function, otherwise it will be parts of [Python API docs](https://spark.apache.org/docs/latest/api/python/pyspark.ml.html#pyspark.ml.clustering.KMeans.setInitMode).
cc mengxr jkbradley

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #10975 from yanboliang/clustering-cleanup.