Commit b9bae02af2 introduced
'last_seen_up' to check service in Liberty. Nova use 'updated_at'
to check that before Liberty. So we can remove legacy item now.
Note: 'last_seen_up' may be null at the beginning. so we still
need 'created_at' in this case.
Change-Id: I5a273c294341027fba66a54a540182fc890522db