Remove unneeded OS_TEST_DBAPI_ADMIN_CONNECTION

The desired OS_TEST_DBAPI_ADMIN_CONNECTION string is now reflected
in the oslo.db 1.12.0 release's default behavior, and so does not
need to be set any longer to achieve the same opportunistic backend
discovery for tests.

Change-Id: I1af372f6f18038e8fc63076b4e399d0ae86f5a23

tox.ini

@@ -30,14 +30,7 @@ commands =
   flake8 {posargs}
 
 [testenv:py34]
-# NOTE(viktors): we must change default connection string for MySQL because
-#                we use a different DB connector (PyMySQL, not MySQLdb) in py3x
-#                env. So we should put new DB URLs in the env variable. This
-#                will allow to run tests, that require MySQL database,
-#                for example DB migration tests.
-setenv =
-    {[testenv]setenv}
-    OS_TEST_DBAPI_ADMIN_CONNECTION=mysql+pymysql://openstack_citest:openstack_citest@localhost/;postgresql://openstack_citest:openstack_citest@localhost/postgres;sqlite://
+setenv = {[testenv]setenv}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =