cmd/internal/gc: remove incorrect "write barrier prohibited" error

Commit 9c9e36b3 pushed these errors down to where the write barriers
are actually emitted, but forgot to remove the original error that was
being pushed down.

Change-Id: I751752a896e78fb9e63d69f88e7fb8d1ff5d344c
Reviewed-on: https://go-review.googlesource.com/10264Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/internal/gc/walk.go

@@ -2206,9 +2206,6 @@ var applywritebarrier_bv Bvec
 
 func applywritebarrier(n *Node, init **NodeList) *Node {
 	if n.Left != nil && n.Right != nil && needwritebarrier(n.Left, n.Right) {
-		if Curfn != nil && Curfn.Func.Nowritebarrier {
-			Yyerror("write barrier prohibited")
-		}
 		if flag_race == 0 {
 			if Debug_wb > 1 {
 				Warnl(int(n.Lineno), "marking %v for barrier", Nconv(n.Left, 0))