Removed a dead db function register_models()
Removed a dead db function register_models()

The actual models are created by the migration modules.
register_models() are no longer used and raises some confusions.

Change-Id: I2f63bb4e7a2d4ace9e9d781f63a33153b61aab94

Hybrid-Cloud/cinder,Datera/cinder,Nexenta/cinder,j-griffith/cinder,j-griffith/cinder,phenoxim/cinder,Hybrid-Cloud/cinder,Nexenta/cinder,mahak/cinder,cloudbase/cinder,eharney/cinder,eharney/cinder,phenoxim/cinder,openstack/cinder,openstack/cinder,cloudbase/cinder,NetApp/cinder,mahak/cinder,Datera/cinder,ge0rgi/cinder,NetApp/cinder