include/mruby/value.h: remove assertion from mrb_obj_value()

The function was called too often, so that even slightest assertion may
bother performance. The function is stable enough.

include/mruby/value.h

@@ -359,8 +359,6 @@ mrb_obj_value(void *p)
 {
   mrb_value v;
   SET_OBJ_VALUE(v, (struct RBasic*)p);
-  mrb_assert(p == mrb_ptr(v));
-  mrb_assert(((struct RBasic*)p)->tt == mrb_type(v));
   return v;
 }