Delete a broken subnet delete unit test

A test to delete a subnet in use was incorrectly
calling 'subnet' instead of 'subnets' in the API request
and asserting a 404 instead of a 409. Even the correct
version of this test is already covered by the
'test_port_prevents_subnet_deletion' method so this
commit just removes the broken test.

Change-Id: I3eac35abbb1d6f8429b4331948aecfc952284219
Merge "Delete a broken subnet delete unit test"