cmd/go/internal/modfetch/codehost: remove invariantly-empty return value from Repo...

cmd/go/internal/modfetch/codehost: remove invariantly-empty return value from Repo.ReadZip

Previously, codehost.Repo.ReadZip returned an 'actualSubdir' value
that was the empty string in all current implementations.

Updates #26092

Change-Id: I6708dd0f13ba88bcf1a1fb405e9d818fd6f9197e
Reviewed-on: https://go-review.googlesource.com/c/go/+/203277
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>