cmd/go/internal/modload: remove outdated comments and redundant tests

The outdated comment in modfile.go was missed in CL 315409.

Upon a closer look at the test case in mod_go_version_vendor.txt, it
is almost completely redundant with the new test in
mod_vendor_goversion.txt. Make it completely redundant and remove it.

Updates #36876

Change-Id: Ibcd1f6b426460aaafbd6dc0be93078547904572b
Reviewed-on: https://go-review.googlesource.com/c/go/+/316209


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