remove a field for debug of union header

include/picrin/gc.h

@@ -11,7 +11,6 @@ union header {
     union header *ptr;
     size_t size;
     enum pic_gc_mark mark;
-    enum pic_tt tt;
   } s;
   long alignment[2];
 };

src/gc.c

@@ -674,7 +674,6 @@ pic_obj_alloc_unsafe(pic_state *pic, size_t size, enum pic_tt tt)
     }
   }
   obj->tt = tt;
-  (((union header *)obj) - 1)->s.tt = tt;
 
   return obj;
 }