Added missing ToList() in RemoveChildGroupsFromOtherGroups

The `RemoveChildGroupFromOtherGroups` method in `GroupService` doesn't have a `ToList()` on the end of `groupRepository.GetByChildID(childGroupID)` which causes "There is already an open DataReader associated with this Command" errors when deleting groups.

The error is caused because the query that retrieves the child IDs is not complete when the call to Update() is made.