project.config: enable auto-submit on release branches

- The label BLOCK on release-managers made no sense and was probably a
  historical accident in c338dc7897d28cad676e64de310ac56c8f05673f.
- But keep gobot's label access, since relui uses it.
- Nothing in the auto-submit code avoids submitting to release branches,
  I think it was just that gobot didn't have submit rights. Grant it.

As of this instant, there are only two open auto-submit CLs and I'm okay
with them getting immediately merged:

https://go-review.git.corp.google.com/q/label:auto-submit+status:open+-branch:master

Change-Id: Ifaf05821029806abf80af76a26f6082ea0d75835
Reviewed-on: https://go-review.googlesource.com/c/All-Projects/+/456618
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>