Drop devstack-gate

The devstack-gate repo has been retired, let's drop all references
to it to avoid errors like this in Zuul:

The project "openstack/devstack-gate" was not found.

Change-Id: I5a792ea53bd65abb23cdaf41fe1994b371c9fc9e

Author: eawever

.zuul.yaml

@@ -23,7 +23,6 @@
 #    timeout: 7800
 #    required-projects:
 #      - openstack/devstack
-#      - openstack/devstack-gate
 #      - recordsansible/ara
 #      - openstack/neutron
 #      - openstack/python-troveclient