cmd/go: remove conversion of legacy pre-module dependency configs

This kind of worked, kind of didn't, but by now no one is running into
those configs anymore during "go mod init", the code is complex,
and the tests are slow. Not worth the trouble of maintaining anymore.

Change-Id: I02d4188d531c68334d17b2462bafec4c5dd49777
Reviewed-on: https://go-review.googlesource.com/c/go/+/518776
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Bypass: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>

Author: rsc