Drop instance_group_metadata from the database

This completes the instance_group metadataectomy started in the previous
patch. Per recent discussions, this decouples the code change from the
actual schema change, allowing them to (potentially) be run at different
times.

Change-Id: I6c3ff35f6e90fbed8b1fd727eb3b0b0ee140abc4
Merge "Drop instance_group_metadata from the database"