Remove unnecessary saving of host and availability_zone
Remove unnecessary saving of host and availability_zone

1. When doing backup creation, we have already used method
   `_get_available_backup_service_host`(in cinder/backup/api.py)
   to check and select availability zone and host, and then
   save them in DB across the API service.
2. When in restore and delete operation, the host of backup
   will never be modified.
So it's safe to remove them from the manager code, since we are not
actually changing the contents. This removes an unnecessary DB
request in backup create, restore, and delete operations.

Change-Id: If6dc705db18e138ed13fe3e9a5eecd366c33fe89

mahak/cinder,openstack/cinder,openstack/cinder,mahak/cinder