Do not allow Project Owners push directly to refs/meta/config

They can use Gerrit like a normal CL, and we're already in the process
of restricting push across the board, so keeping this here will just
confuse people.

Change-Id: I68a0e658e74a344bd709cadb039f43a2b4da59ae
Reviewed-on: https://chromium-review.googlesource.com/c/All-Projects/+/5544742
Reviewed-by: Joey Scarr <jsca@chromium.org>
Reviewed-by: Scott Lee <ddoman@chromium.org>