[SPARK-11975][ML] Remove duplicate mllib example (DT/RF/GBT in Java/Python)

Remove duplicate mllib example (DT/RF/GBT in Java/Python).
Since we have tutorial code for DT/RF/GBT classification/regression in Scala/Java/Python and example applications for DT/RF/GBT in Scala, so we mark these as duplicated and remove them.
mengxr

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #9954 from yanboliang/SPARK-11975.