Remove unused global variable from unit tests

DEFAULT_TEST_DB_FILE was assigned but never used. We don't seem to have any
documentation around it's purpose so I'm guess it was left over from a
refactor.

Change-Id: Ic8fab5399b9e54ec91a90b2ac12a82f48490949a

keystone/tests/unit/core.py

@@ -111,10 +111,6 @@ class dirs(object):
         return os.path.join(TESTCONF, *p)
 
 
-# keystone.common.sql.initialize() for testing.
-DEFAULT_TEST_DB_FILE = dirs.tmp('test.db')
-
-
 class EggLoader(loadwsgi.EggLoader):
     _basket = {}