decoder/Control: remove attribute client_is_waiting

This optimization is useless because sane pthread_cond_signal()
implementations check the number of waiters and do not invoke a system
call if there are none.

src/decoder/Control.hxx

@@ -111,13 +111,6 @@ private:
 	bool quit;
 
 public:
-	/**
-	 * Is the client currently waiting for the DecoderThread?  If
-	 * false, the DecoderThread may omit invoking Cond::signal(),
-	 * reducing the number of system calls.
-	 */
-	bool client_is_waiting = false;
-
 	bool seek_error;
 	bool seekable;
 	SongTime seek_time;