[SPARK-3381] [MLlib] Eliminate bins for unordered features in DecisionTrees

For unordered features, it is sufficient to use splits since the threshold of the split corresponds the threshold of the HighSplit of the bin and there is no use of the LowSplit.

Author: MechCoder <manojkumarsivaraj334@gmail.com>

Closes #4231 from MechCoder/spark-3381 and squashes the following commits:

58c19a5 [MechCoder] COSMIT
c274b74 [MechCoder] Remove unordered feature calculation in labeledPointToTreePoint
b2b9b89 [MechCoder] COSMIT
d3ee042 [MechCoder] [SPARK-3381] [MLlib] Eliminate bins for unordered features