Bug 1462791 - Remove docker_worker_add_gecko_vcs_env_vars(); r=dustin

support_vcs_checkout() always sets the environment variables that
were set by this deleted function. In addition, support_vcs_checkout()
also adds caches and scopes - at least for docker-worker and
docker-engine. For generic-worker - which was used in all call sites
of docker_worker_add_gecko_vcs_env_vars() (yes, the "docker_worker"
bit of the name was completely wrong - probably a legacy holdover) -
support_vcs_checkout() was *almost* exactly equivalent to
docker_worker_add_gecko_vcs_env_vars(). The only difference is that
support_vcs_checkout() adds the
secrets:get:project/taskcluster/gecko/hgfingerprint scope in addition
to setting the environment variables.

MozReview-Commit-ID: 8fl3u9be5fT