runtime: remove declaration of function that does not exist

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/10730045

src/pkg/runtime/proc.c

@@ -87,7 +87,6 @@ static void procresize(int32);
 static void acquirep(P*);
 static P* releasep(void);
 static void newm(void(*)(void), P*);
-static void goidle(void);
 static void stopm(void);
 static void startm(P*, bool);
 static void handoffp(P*);