[API]: Handle no device attached on delete

If no device was attached to the LB during a delete (manual deletion inside
MySQL happened) the LB would be stuck in PENDING_DELETE forever and an
exception would occur in the logs.

Now when this case is detected we just mark the LB as DELETED.

Change-Id: I2b420b46cce9f51294464a58637e24a140744c58
Merge "[API]: Handle no device attached on delete"