Revert "[SPARK-7212] [MLLIB] Add sequence learning flag"

This reverts commit 25f574eb. After speaking to some users and developers, we realized that FP-growth doesn't meet the requirement for frequent sequence mining. PrefixSpan (SPARK-6487) would be the correct algorithm for it. feynmanliang

Author: Xiangrui Meng <meng@databricks.com>

Closes #7240 from mengxr/SPARK-7212.revert and squashes the following commits:

2b3d66b [Xiangrui Meng] Revert "[SPARK-7212] [MLLIB] Add sequence learning flag"