cmd/go/internal/modload: revert duplicate path suppression in modload.ImportMissingError

cmd/go/internal/modload: revert duplicate path suppression in modload.ImportMissingError

CL 210338 suppressed duplication for import paths mentioned in an
ImportMissingError.

Unfortunately, that broke one of the cases in
cmd/go/internal/modload.TestImport, and the new error message is still
kind of awkward anyway.

Let's revert that part of the change — we can try again with more
coverage for that case.

Updates #35986

Change-Id: Ib0858aec4f89a7231e32c35ec876da80d80f2098
Reviewed-on: https://go-review.googlesource.com/c/go/+/210342
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>