Remove the Ocata stable branch jobs from master gate

Tempest 20.0.0 is released now, which marks the end of support
for Ocata release - https://review.openstack.org/#/c/645867/

We do not need to run the Ocata stable job om master gate now.
Depends-On: https://review.openstack.org/#/c/645867

Change-Id: I883f3c98201631acdadeaab6678a796a85662e44
1 file changed