Delete APEX CE snapshots when a rollbck is deleted (2/n)

Now we delete APEX CE snapshots immediately when a rollback is deleted.
If the user is locked, the deletion will fail, but that's OK. The
orphaned snapshots will be cleaned when the user is unlocked.

Bug: 158712947
Test: atest StagedRollbackTest RollbackUnitTest
Change-Id: I3e3eef4c52f281d28a6a6ac6735c27ef8b83ddc2