Revert management cmd

I didn't end up using this for [Tim-226](https://edx-wiki.atlassian.net/browse/TIM-226) and currently it does not work (because it does not set the XBlock's `submission_uuid` field).  In the interest of keeping the code base clean, I'd like to remove it.  If we need something similar later, we can look back to the original PR's branch as a starting point.