ci: drop gcloud init from ci steps

Created by: jhchabran

Following up on INC-25 we don't need anymore to explicitly call gcloud auth configure-docker (as it's plausibly causing racing access to ~/.docker/config.json) because this authentication step is now done when the agents are started.

See the relevant PR on the sourcegraph/infrastructure repository: https://github.com/sourcegraph/infrastructure/pull/2829

I'm navigating murky waters with this pull request, see my comments on the specific changes which I am unsure of.

Nevertheless, this set of changes produces green builds as you can see here: https://buildkite.com/sourcegraph/sourcegraph/builds/113188#a30c21f0-4a7d-47d8-83d0-183e72abbac3