Use upstream whatsapp again (#809)

vendor/golang.org/x/crypto/poly1305/sum_noasm.go

@@ -10,5 +10,7 @@ package poly1305
 // 16-byte result into out. Authenticating two different messages with the same
 // key allows an attacker to forge messages at will.
 func Sum(out *[TagSize]byte, msg []byte, key *[32]byte) {
-	sumGeneric(out, msg, key)
+	h := newMAC(key)
+	h.Write(msg)
+	h.Sum(out)
 }