message: Remove unused headers from scheduled actor
diff: #include "caf/stream_result.hpp"
  #include "caf/response_handle.hpp"
  #include "caf/scheduled_actor.hpp"
  -#include "caf/stream_sink_impl.hpp"
  -#include "caf/stream_stage_impl.hpp"
  -#include "caf/stream_source_impl.hpp"
  #include "caf/stream_result_trait.hpp"
  -#include "caf/terminal_stream_scatterer.hpp"
  #include "caf/to_string.hpp"
  