Reduce the size of struct RBreak to reduce memory usage.

The old size of `struct RBreak` was 56 bytes (`MRB_NO_BOXING`)
and it's bigger than other object structures. That increase the
size of `RVALUE` thus increase the total amount of memory consumption.

include/mruby/error.h

@@ -34,7 +34,6 @@ MRB_API mrb_value mrb_f_raise(mrb_state*, mrb_value);
 
 struct RBreak {
   MRB_OBJECT_HEADER;
-  struct iv_tbl *iv;
   struct RProc *proc;
   mrb_value val;
 };

src/vm.c

@@ -810,7 +810,6 @@ break_new(mrb_state *mrb, struct RProc *p, mrb_value val)
   struct RBreak *brk;
 
   brk = (struct RBreak*)mrb_obj_alloc(mrb, MRB_TT_BREAK, NULL);
-  brk->iv = NULL;
   brk->proc = p;
   brk->val = val;