cmd/gc: remove "send used as value" hint

This message was helpful for pre-Go 1 users updating to Go 1.
That time is past. Now the message is confusing because it
depends on knowing what pre-Go 1 looked like.

Update #4697.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/13335051

src/cmd/gc/typecheck.c

@@ -928,11 +928,7 @@ reswitch:
 		goto ret;
 
 	case OSEND:
-		if(top & Erv) {
-			yyerror("send statement %N used as value; use select for non-blocking send", n);
-			goto error;
-		}
-		ok |= Etop | Erv;
+		ok |= Etop;
 		l = typecheck(&n->left, Erv);
 		typecheck(&n->right, Erv);
 		defaultlit(&n->left, T);