main.star: grant commit rights to approvers

CQ+2 is still disabled for most people, so the main change here is
allowing approvers to trigger trybots for other people's CLs.

Also remove rendundant groups -- mdb/golang is a member of approvers is
a member of may-start-trybots.

Change-Id: I6225f48c89705a5751f7031b1e208aa1968725f5
Reviewed-on: https://go-review.googlesource.com/c/build/+/522236
TryBot-Bypass: Heschi Kreinick <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Heschi Kreinick <[email protected]>

Author: heschi