cmd/go/testdata: remove directory badmod

It doesn't seem to be used anywhere.

Change-Id: If979b6d69c9bf5261afd980d96f67991c417d46d
Reviewed-on: https://go-review.googlesource.com/c/go/+/213218
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/testdata/badmod/go.mod

-module m

src/cmd/go/testdata/badmod/x.go

-package x
-
-import _ "appengine"
-import _ "nonexistent.rsc.io" // domain does not exist