runtime: remove unused var

LGTM=bradfitz
R=daniel.morsing, bradfitz
CC=golang-codereviews
https://golang.org/cl/130500044

src/pkg/runtime/proc.c

@@ -1868,7 +1868,6 @@ newproc_m(void)
 	void *callerpc;
 	FuncVal *fn;
 	int32 siz;
-	G *spawng;
 
 	siz = g->m->scalararg[0];
 	callerpc = (void*)g->m->scalararg[1];