zuul: Remove dedicated devstack-async job

I83d56c9363d481bb6d5921f5e1f9b024f136044b switched the default of
DEVSTACK_PARALLEL over to True so this dedicated job is no longer
required as *all* jobs should now be using it.

Change-Id: I0f475ab177c2cd49eeb6be861cdd11581e8e0b97
1 file changed