message: Remove unused flow labels
diff: @@ -40,18 +40,8 @@ namespace caf {
  /// Stream communication messages for handshaking, ACKing, data transmission,
  /// etc.
  struct stream_msg : tag::boxing_type {
  - /// A flow label characterizes nested types.
  - enum flow_label {
  - /// Identifies content types that only flow downstream.
  - flows_downstream,
  - /// Identifies content types that only flow upstream.
  - flows_upstream
  - };
  -
  /// Initiates a stream handshake.
  struct open {
  - /// Allows visitors to dispatch on this tag.
  - static constexpr flow_label label = flows_downstream;
  /// Allows the testing DSL to unbox this type automagically.
  using outer_type = stream_msg;
  /// Contains a type-erased stream<T> object as first argument followed by
  @@ -70,8 +60,6 @@ struct stream_msg : tag::boxing_type {
  /// Acknowledges a previous `open` message and finalizes a stream handshake.
  /// Also signalizes initial demand.
  struct ack_open {
  - /// Allows visitors to dispatch on this tag.
  - static constexpr flow_label label = flows_upstream;
  /// Allows the testing DSL to unbox this type automagically.
  using outer_type = stream_msg;
  /// Allows actors to participate in a stream instead of the actor
  @@ -89,8 +77,6 @@ struct stream_msg : tag::boxing_type {
  /// Transmits stream data.
  struct batch {
  - /// Allows visitors to dispatch on this tag.
  - static constexpr flow_label label = flows_downstream;
  /// Allows the testing DSL to unbox this type automagically.
  using outer_type = stream_msg;
  /// Size of the type-erased vector<T> (used credit).
  @@ -104,8 +90,6 @@ struct stream_msg : tag::boxing_type {
  /// Cumulatively acknowledges received batches and signalizes new demand from
  /// a sink to its source.
  struct ack_batch {
  - /// Allows visitors to dispatch on this tag.
  - static constexpr flow_label label = flows_upstream;
  /// Allows the testing DSL to unbox this type automagically.
  using outer_type = stream_msg;
  /// Newly available credit.
  @@ -116,24 +100,18 @@ struct stream_msg : tag::boxing_type {
  /// Orderly shuts down a stream after receiving an ACK for the last batch.
  struct close {
  - /// Allows visitors to dispatch on this tag.
  - static constexpr flow_label label = flows_downstream;
  /// Allows the testing DSL to unbox this type automagically.
  using outer_type = stream_msg;
  };
  /// Informs a source that a sink orderly drops out of a stream.
  struct drop {
  - /// Allows visitors to dispatch on this tag.
  - static constexpr flow_label label = flows_upstream;
  /// Allows the testing DSL to unbox this type automagically.
  using outer_type = stream_msg;
  };
  /// Propagates a fatal error from sources to sinks.
  struct forced_close {
  - /// Allows visitors to dispatch on this tag.
  - static constexpr flow_label label = flows_downstream;
  /// Allows the testing DSL to unbox this type automagically.
  using outer_type = stream_msg;
  /// Reason for shutting down the stream.
  @@ -142,8 +120,6 @@ struct stream_msg : tag::boxing_type {
  /// Propagates a fatal error from sinks to sources.
  struct forced_drop {
  - /// Allows visitors to dispatch on this tag.
  - static constexpr flow_label label = flows_upstream;
  /// Allows the testing DSL to unbox this type automagically.
  using outer_type = stream_msg;
  /// Reason for shutting down the stream.
  