Drop DEVSTACK_GATE_FEATURE_MATRIX

DEVSTACK_GATE_FEATURE_MATRIX seems to be an old legacy thing that
is no longer being used.  It currently prevents using the jobs in
openstack/devstack without adding openstack/devstack-gate for the
role.

Change-Id: Iab9b4862c01043d2c158398bac4b3b289a0adba0
2 files changed