Remove: "language" field from server/client

The original idea was that people could find a server they could
talk in their native language on. This isn't really used in that
way. There are several reasons for removing this:

- the client also sends his "language" to the server, but nothing
  is doing anything with this.
- flags are a bad way to represent languages, and over the years
  we had several (rightfully) complaints about this.
- most servers have their language set to "All", and prefix the
  servername with the language it is about. This is a much more
  efficient way to do the same.

All in all, this feature should go back to the drawing board.
Maybe it could work in another form, but this form is not it.

src/settings_type.h

@@ -280,7 +280,6 @@ struct NetworkSettings {
 	uint8  max_spectators;                                ///< maximum amount of spectators
 	Year   restart_game_year;                             ///< year the server restarts
 	uint8  min_active_clients;                            ///< minimum amount of active clients to unpause the game
-	uint8  server_lang;                                   ///< language of the server
 	bool   reload_cfg;                                    ///< reload the config file before restarting
 	char   last_host[NETWORK_HOSTNAME_LENGTH];            ///< IP address of the last joined server
 	uint16 last_port;                                     ///< port of the last joined server