Revert "Use DraftMongoModuleStore."

This reverts commit bd33e9a34ab63047fd56c52a1d5ba09f3267cd45.

This is to maintain backwards compatibility with older versions
of edx-platform.
Merge pull request #1338 from edx/feanil/revert_modulestore_changes_rel

Revert "Use DraftMongoModuleStore."