Remove quota_class db API calls

Previous patches:
* Remove quota_class API I1110022d6f628d03aaf363da707f2d2ef1600437
* Removed the quota_class from nova.context in I85dd24121a434b95dc6f0af180444d4258b50765
* Removed quota_class from nova.quotas in I1e788414f2f86d846612d3c4e909caae46273f4e

Now that we aren't using quota_class anywhere, remove the DB API. A
subsequent patch will add the database migration to drop the table.

Change-Id: Idd888cc45533140d500fbe0bca1fed36d6fb0274
Merge "Remove quota_class db API calls"