Unflag Group Configurations; Render only existing children in staff view of split_test.

This fixes 

  *  [BLD-1224](https://openedx.atlassian.net/browse/BLD-1224) Error split test rendering in LMS
  * [BLD-1229](https://openedx.atlassian.net/browse/BLD-1229)  Do not hide Group Configuration editor behind feature flag

Root cause for BLD-1224 is that group_id_to_child contains deleted groups.
Other solution is to clean group_id_to_child of split_test when group was deleted from group configuration page. But this will affect publishing. So this solution was implemented but skipped.

@tymofij @cahrens please review.

Also this provides fix for https://openedx.atlassian.net/browse/BLD-1249.