Remove refs.cfg which is no longer used.

Change-Id: I9e5c5c12bc4c37d1aded50a8c230c1db3fca2587
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1815458
Auto-Submit: Garrett Beaty <gbeaty@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Garrett Beaty <gbeaty@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700346}

infra/config/refs.cfg

-# Refs configuration file. The documentation of the format can be found
-# at https://luci-config.appspot.com/schemas/projects:refs.cfg.
-refs {
-  name: "refs/heads/master"
-  config_path: "infra/config/branch"
-}