Remove unneeded OS_TEST_DBAPI_ADMIN_CONNECTION
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: I9e7239c71371bc872f5ae41e4f0f3c8942f3d9b3

scality/manila,sajuptpm/manila,vponomaryov/manila,scality/manila,weiting-chen/manila,jcsp/manila,openstack/manila,vponomaryov/manila,bswartz/manila,NetApp/manila,redhat-openstack/manila,jcsp/manila,redhat-openstack/manila,sajuptpm/manila,weiting-chen/manila,NetApp/manila,openstack/manila,bswartz/manila