do not support song format for command that is executed on song change

doc/config

@@ -141,9 +141,11 @@
 ##### various settings #####
 #
 ##
-## Note: Custom command that will be executed each time song changes.
-## It uses the same syntax as song format (for now playing song).
-## Useful for notifications etc.
+## Note: Custom command that will be executed each
+## time song changes. Useful for notifications etc.
+##
+## Attention: It doesn't support song format anymore.
+## Use `ncmpcpp --now-playing SONG_FORMAT` instead.
 ##
 #execute_on_song_change = ""
 #

src/status.cpp

@@ -368,7 +368,7 @@ void NcmpcppStatusChanged(Connection *, StatusChanges changed, void *)
 			np = Mpd.GetCurrentSong();
 			
 			if (!Config.execute_on_song_change.empty())
-				system(np.toString(Config.execute_on_song_change).c_str());
+				system(Config.execute_on_song_change.c_str());
 			if (Mpd.GetState() > psStop)
 				WindowTitle(utf_to_locale_cpy(np.toString(Config.song_window_title_format)));
 			if (Config.autocenter_mode && !myPlaylist->Main()->isFiltered())