Revert "net: permit use of Resolver.PreferGo, netgo on Windows and Plan 9"

Revert "net: permit use of Resolver.PreferGo, netgo on Windows and Plan 9"

This reverts CL 400654.

Reason for revert: broke net.TestGoLookupIP on the darwin-amd64-nocgo builder.

Updates #33097.

Change-Id: Idaf94eda88c9d4401e667a4d31c00ce376d91909
Reviewed-on: https://go-review.googlesource.com/c/go/+/401754
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>