remove "lobby sounds" advanced preference

It is defunct now, having been replaced by the lobby sounds options
dialogs.

data/advanced_preferences.cfg

@@ -108,13 +108,6 @@
     default=yes
 [/advanced_preference]
 
-[advanced_preference]
-    field=lobby_sounds
-    name= _ "Lobby sounds"
-    type=boolean
-    default=yes
-[/advanced_preference]
-
 [advanced_preference]
     field=desktop_notifications
     name= _ "Desktop notifications"

src/gui/dialogs/lobby_main.cpp

@@ -318,7 +318,6 @@ void tlobby_main::append_to_chatbox(const std::string& text, size_t id)
 
 void tlobby_main::do_notify(t_notify_mode mode)
 {
-	if(preferences::lobby_sounds()) {
 		switch(mode) {
 			case NOTIFY_WHISPER:
 			case NOTIFY_WHISPER_OTHER_WINDOW:
@@ -343,7 +342,6 @@ void tlobby_main::do_notify(t_notify_mode mode)
 			default:
 				break;
 		}
-	}
 }
 
 tlobby_main::tlobby_main(const config& game_config,

src/lobby_preferences.cpp

@@ -19,11 +19,6 @@
 
 namespace preferences {
 
-bool lobby_sounds()
-{
-	return preferences::get("lobby_sounds", true);
-}
-
 bool sort_list()
 {
 	return preferences::get("sort_list", true);

src/lobby_preferences.hpp

@@ -19,8 +19,6 @@
 
 namespace preferences {
 
-	bool lobby_sounds();
-
 	bool sort_list();
 	void _set_sort_list(bool show);