Remove unused code in api/compute/admin/test_quotas.py

The 'tenants' variable is not used anywhere in the class, which should be
removed. Maybe it was ignored by someone when fixing something else.

Change-Id: I96eb4ff9590c6416c21d457e93075ddc74e5f3f3
Closes-bug: #1249804
Merge "Remove unused code in api/compute/admin/test_quotas.py"