Remove refs.cfg.

After submitting the last CL I received an email with the note:

Some configs in revision 780c594 in tint repo have validation errors.
luci-config had to reject all configs in this revision.

  * ERROR: refs.cfg: refs.cfg is not used since 2019 and must be deleted

Change-Id: I40c4fcdccd9af53aa47e32f074065782876f179d
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/25080
Reviewed-by: Ryan Harrison <rharrison@chromium.org>
diff --git a/infra/config/global/refs.cfg b/infra/config/global/refs.cfg
deleted file mode 100644
index 6f1e356..0000000
--- a/infra/config/global/refs.cfg
+++ /dev/null
@@ -1,6 +0,0 @@
-# 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/main"
-  config_path: "infra/config/branch"
-}