main.star: clean up after dropping TOOLS_GO_BRANCHES

Delete code that became dead after CL 765501. Generated files unchanged.

For golang/go#69321.

Change-Id: I51030041ad183930fc733b1c33ea90876bc58c79
Reviewed-on: https://go-review.googlesource.com/c/build/+/765503
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>