[dev.cc] cmd/gc: fix warning on Plan 9

warning: src/cmd/gc/walk.c:1769 set and not used: on

LGTM=rsc
R=rsc, minux
CC=golang-codereviews
https://golang.org/cl/175850043

src/cmd/gc/walk.c

@@ -1766,7 +1766,6 @@ walkprint(Node *nn, NodeList **init)
 	int notfirst, et, op;
 	NodeList *calls;
 
-	on = nil;
 	op = nn->op;
 	all = nn->list;
 	calls = nil;