project.config: remove obsolete security branch permissions

Dmitri says these were an artifact of the old security release process; no such branches exist
today. The new security process is golang/go#44918.

Change-Id: I186c4939731f0cf76109d8867c465e9f8b27a4f0
Reviewed-on: https://go-review.googlesource.com/c/All-Projects/+/456615
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>