Revert "Clear `irep->outer` when no `Proc` reference the `irep`; fix #3793"

This reverts commit 15d48efa.

src/gc.c

@@ -815,11 +815,7 @@ obj_free(mrb_state *mrb, struct RBasic *obj, int end)
       struct RProc *p = (struct RProc*)obj;
 
       if (!MRB_PROC_CFUNC_P(p) && p->body.irep) {
-        int refcnt = p->body.irep->refcnt;
         mrb_irep_decref(mrb, p->body.irep);
-        if (refcnt == 2) {    /* reference only from irep */
-          p->body.irep->outer = NULL;
-        }
       }
     }
     break;