remove tearDownClass in test_quotas.py

There is no need for the functionality of the tearDownClass in the
api/compute/admin/test_quotas.py, becase it's already included in the base
class. So it should be removed.

Change-Id: Ic0e145952cc143d65291258fc9567807ca051596
closes-bug: #1239722
Merge "remove tearDownClass in test_quotas.py"