cmd/8g: remove unused variable (fix build)

http://build.golang.org/log/0434a945e3351eedaf56aa824d2bfe9c0d5e6735

LGTM=dave
R=bradfitz, dave
CC=golang-codereviews
https://golang.org/cl/144800043

src/cmd/8g/ggen.c

@@ -210,7 +210,6 @@ clearfat(Node *nl)
 void
 ginscall(Node *f, int proc)
 {
-	int32 arg;
 	Prog *p;
 	Node reg, r1, con;