message: Remove redundant type alias
diff: @@ -188,9 +188,6 @@ public:
  /// A queue optimized for single-reader-many-writers.
  using mailbox_type = intrusive::fifo_inbox<mailbox_policy>;
  - /// A reference-counting pointer to a `stream_manager`.
  - using stream_manager_ptr = intrusive_ptr<stream_manager>;
  -
  /// The message ID of an outstanding response with its callback.
  using pending_response = std::pair<const message_id, behavior>;
  