cmd/go: remove global loader state variable

cmd/go: remove global loader state variable

Prior commits have removed all dependencies on the global module
loader state variable, so it is now unnecessary and thus removed.

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

Change-Id: Idaf033ee703a18ffd29e40fad80ef13c7b33dbec
Reviewed-on: https://go-review.googlesource.com/c/go/+/711139
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>