Remove setting of DEVSTACK_GATE_EXERCISES
Remove setting of DEVSTACK_GATE_EXERCISES

This defaults to 0 anyway, and is being removed as the devstack
excercises are being removed. see https://review.openstack.org/#/c/581377/

Change-Id: I192b5bb5f59f235170f020a51fa1afd86887b6b6

openstack/neutron-fwaas,openstack/neutron-fwaas