[SPARK-14903][SPARK-14071][ML][PYTHON] Revert : MLWritable.write property

## What changes were proposed in this pull request?

SPARK-14071 changed MLWritable.write to be a property.  This reverts that change since there was not a good way to make MLReadable.read appear to be a property.

## How was this patch tested?

existing unit tests

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #12671 from jkbradley/revert-MLWritable-write-py.