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: Ie955bd170698dd04924b2823b0c38b585e1892b3

openstack/heat,openstack/heat