ActivityPub: Add back debug call + explanation.

lib/pleroma/web/activity_pub/activity_pub.ex

@@ -1378,6 +1378,11 @@ def fetch_and_prepare_user_from_ap_id(ap_id, opts \\ []) do
          {:ok, data} <- user_data_from_user_object(data) do
       {:ok, maybe_update_follow_information(data)}
     else
+      # If this has been deleted, only log a debug and not an error
+      {:error, "Object has been deleted" = e} ->
+        Logger.debug("Could not decode user at fetch #{ap_id}, #{inspect(e)}")
+        {:error, e}
+
       {:error, {:reject, reason} = e} ->
         Logger.info("Rejected user #{ap_id}: #{inspect(reason)}")
         {:error, e}