cmd/go/internal/str: simplify HasPathPrefix by epsilon

Pointed out in CL 122396.
An empty prefix has already been handled above.

Change-Id: Ib94df0a9c8c0517f932b90126232111caa9ad289
Reviewed-on: https://go-review.googlesource.com/124797Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/str/path.go

@@ -19,7 +19,7 @@ func HasPathPrefix(s, prefix string) bool {
 		return true
 	}
 	if len(s) > len(prefix) {
-		if prefix != "" && prefix[len(prefix)-1] == '/' || s[len(prefix)] == '/' {
+		if prefix[len(prefix)-1] == '/' || s[len(prefix)] == '/' {
 			return s[:len(prefix)] == prefix
 		}
 	}