cmd/go: removed unused code in toolchain.Exec

cmd/go: removed unused code in toolchain.Exec

This change removes the unused module loader state in the function
`toolchain.Exec`.

This commit is part of the overall effort to eliminate global
modloader state.

Change-Id: I8935f14447db4669457becc5a96db7f45132772f
Reviewed-on: https://go-review.googlesource.com/c/go/+/709980
Reviewed-by: Michael Matloob <matloob@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>