Remove unused db api methods

Remove methods that are no longer used from the db API.  New methods
were created that replaced these when the EC2SnapshotMapping object
was created in earlier commits.

Related to blueprint convert-ec2-api-to-use-nova-objects

Change-Id: I5431dd34d6a3fcea687b9d8f192a822e83ed9402
Merge "Remove unused db api methods"