Automatic merge from submit-queue (batch tested with PRs 38111, 38121)

remove rbac super user

Cleaning up cruft and duplicated capabilities as we transition from RBAC alpha to beta.  In 1.5, we added a secured loopback connection based on the `system:masters` group name.  `system:masters` have full power in the API, so the RBAC super user is superfluous.

The flag will stay in place so that the process can still launch, but it will be disconnected.

@kubernetes/sig-auth