Make ObjectSpace::WeakMap#[]= return Qnil

Instead of returning some obscure value in ObjectSpace::WeakMap#[]= that's
not even useful, we can just simply return Qnil.

weakmap.c

@@ -439,18 +439,6 @@ wmap_values(VALUE self)
     return ary;
 }
 
-static VALUE
-nonspecial_obj_id(VALUE obj)
-{
-#if SIZEOF_LONG == SIZEOF_VOIDP
-    return (VALUE)((SIGNED_VALUE)(obj)|FIXNUM_FLAG);
-#elif SIZEOF_LONG_LONG == SIZEOF_VOIDP
-    return LL2NUM((SIGNED_VALUE)(obj) / 2);
-#else
-# error not supported
-#endif
-}
-
 static int
 wmap_aset_replace(st_data_t *key, st_data_t *val, st_data_t new_key_ptr, int existing)
 {
@@ -495,7 +483,7 @@ wmap_aset(VALUE self, VALUE key, VALUE val)
     RB_OBJ_WRITTEN(self, Qundef, key);
     RB_OBJ_WRITTEN(self, Qundef, val);
 
-    return nonspecial_obj_id(val);
+    return Qnil;
 }
 
 /* Retrieves a weakly referenced object with the given key */