cmd/go/internal/modfetch: stop migrating go.modverify to go.sum

go.modverify was renamed to go.sum before vgo was merged into
cmd/go. It's been long enough that we can safely drop support for it.

For #25525

Change-Id: If8da66280a0fb6a4d4db0b170700775523c18571
Reviewed-on: https://go-review.googlesource.com/c/go/+/240458


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>