Revert "ci: Switch over to an autoscaling GKE cluster for builds."

Revert "ci: Switch over to an autoscaling GKE cluster for builds."

This reverts commit c9df92bf795af878c38538c85f781291c78ec513.

It turns out that gitlab-runner uses kubernetes all wrong, spawning Pods
and sshing into them to run the script instead of Jobs containing the
script to run.  This means that when anything goes wrong with the pod
(autoscale, preemption, VM maintenance, cluster reconfiguration), the job
fails and only sometimes gets handled as a runner system failure.  Even
worse, due to bugs in either the runner or k8s itself, some classes of
timeout-related failure end up not being reported as failures, and the job
will incorrectly report success!

Disable using the "autoscale" cluster until we can do something else
(docker-machine instead of k8s, or the custom third-party k8s-native
runner).

Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Acked-by: Daniel Stone <daniels@collabora.com>