project.config: remove use of release-managers

mdb/golang-release-eng-policy is a better-maintained superset, and should be used where we want people to have ambient access. Remove ambient branch and tag creation permissions -- these are dangerous things that don't happen enough to justify ambient access.

Change-Id: I55499416f24cf5f280c0ed3d9fc0b4f0150944b3
Reviewed-on: https://go-review.googlesource.com/c/All-Projects/+/456619
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>