pull: Drop obsoleted "related objects" API

We removed support for writing "related objects" from ostree commits
in ostree git c9b61cbfee because it just
didn't work out as an idea.  This also removes the API and code from
"ostree pull".

Note there was no test suite coverage.

https://bugzilla.gnome.org/show_bug.cgi?id=706342

src/libostree/ostree-repo-pull.c

@@ -747,23 +747,6 @@ scan_commit_object (OtPullData         *pull_data,
                                  cancellable, error))
     goto out;
   
-  if (pull_data->flags & OSTREE_REPO_PULL_FLAGS_RELATED)
-    {
-      const char *name;
-      gs_unref_variant GVariant *csum_v = NULL;
-
-      related_objects = g_variant_get_child_value (commit, 2);
-      iter = g_variant_iter_new (related_objects);
-
-      while (g_variant_iter_loop (iter, "(&s@ay)", &name, &csum_v))
-        {
-          if (!scan_one_metadata_object (pull_data, ostree_checksum_bytes_peek (csum_v),
-                                         OSTREE_OBJECT_TYPE_COMMIT, recursion_depth + 1,
-                                         cancellable, error))
-            goto out;
-        }
-    }
-
   ret = TRUE;
  out:
   if (iter)