Remove mrb_checkstack() as unused.

include/mruby.h

@@ -209,7 +209,6 @@ mrb_state* mrb_open(void);
 mrb_state* mrb_open_allocf(mrb_allocf, void *ud);
 void mrb_irep_free(mrb_state*, struct mrb_irep*);
 void mrb_close(mrb_state*);
-int mrb_checkstack(mrb_state*,int);
 
 mrb_value mrb_top_self(mrb_state *);
 mrb_value mrb_run(mrb_state*, struct RProc*, mrb_value);

src/vm.c

@@ -170,13 +170,6 @@ stack_extend(mrb_state *mrb, int room, int keep)
   }
 }
 
-int
-mrb_checkstack(mrb_state *mrb, int size)
-{
-  stack_extend(mrb, size+1, 1);
-  return 0;
-}
-
 struct REnv*
 uvenv(mrb_state *mrb, int up)
 {