[SPARK-20670][ML] Simplify FPGrowth transform

## What changes were proposed in this pull request?

jira: https://issues.apache.org/jira/browse/SPARK-20670
As suggested by Sean Owen in https://github.com/apache/spark/pull/17130, the transform code in FPGrowthModel can be simplified.

As I tested on some public dataset http://fimi.ua.ac.be/data/, the performance of the new transform code is even or better than the old implementation.

## How was this patch tested?

Existing unit test.

Author: Yuhao Yang <yuhao.yang@intel.com>

Closes #17912 from hhbyyh/fpgrowthTransform.