cmd/go/internal/modfetch: remove redundant check for semver
cmd/go/internal/modfetch: remove redundant check for semver

The function semver.IsValid(vers) would check whether vers has a "v" prefix.

Change-Id: I9d8381af6bd04d228846701b3f910ec5add61ae6
Reviewed-on: https://go-review.googlesource.com/118936
Run-TryBot: Baokun Lee <34bec6869484f0df7ece4af082d6538ebafb3899@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

golang/vgo,golang/vgo