main.star: revert "enable golang.org/x trybot coverage for supported Go versions"

The approach I've attempted doesn't work¹, so revert the logical
change applied in CL 521935 while keeping some style cleanups.

¹ https://go-review.googlesource.com/c/mod/+/500335/1#message-562d27f528aa647d2efa8c7fb9a0886ddc3e8519

Change-Id: Idd08ef2b54d7121dd0c00eb334a24aad44862559
Reviewed-on: https://go-review.googlesource.com/c/build/+/522238
Reviewed-by: Heschi Kreinick <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
TryBot-Bypass: Dmitri Shuralyov <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>

Author: dmitshur