Consider removing ambiguous HEAD support in the future (once k8s.sgdev.org is fixed)

Created by: slimsag

Support for handling ambiguous HEAD revisions (where a very bad repository contains BOTH a HEAD branch and a remotes/origin/HEAD -> master reference) was added in https://github.com/sourcegraph/sourcegraph/pull/4617 primarily for testing the feature on k8s.sgdev.org, and we should consider removing it in the future.

See https://sourcegraph.slack.com/archives/CHEKCRWKV/p1561004415100200?thread_ts=1560992541.075000&cid=CHEKCRWKV for context.