src: remove unimplemented method in class StreamPipe

PR-URL: https://github.com/nodejs/node/pull/26202
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/stream_pipe.h

@@ -27,7 +27,6 @@ class StreamPipe : public AsyncWrap {
   inline StreamBase* sink();
 
   inline void ShutdownWritable();
-  inline void FlushToWritable();
 
   bool is_reading_ = false;
   bool is_writing_ = false;