Remove redundant use of asio::coroutine in flat_stream

Signed-off-by: Damian Jarek <damian.jarek93@gmail.com>

include/boost/beast/core/impl/flat_stream.hpp

@@ -16,7 +16,6 @@
 #include <boost/beast/core/stream_traits.hpp>
 #include <boost/beast/websocket/teardown.hpp>
 #include <boost/asio/buffer.hpp>
-#include <boost/asio/coroutine.hpp>
 #include <memory>
 
 namespace boost {
@@ -30,7 +29,6 @@ template<class Handler>
 class write_op
     : public async_base<Handler,
         beast::executor_type<flat_stream>>
-    , public net::coroutine
 {
 public:
     template<