Game Launcher: removed set_tutorial

Since 4282d0b the tutorial no longer has a dedicated titlescreen button,
which was the only use of this.

src/game_launcher.cpp

@@ -738,17 +738,6 @@ bool game_launcher::load_game()
 	return true;
 }
 
-void game_launcher::set_tutorial()
-{
-	state_.clear();
-	state_.classification().campaign_type = game_classification::CAMPAIGN_TYPE::TUTORIAL;
-	state_.classification().campaign_define = "TUTORIAL";
-	state_.classification().campaign = "Tutorial";
-	state_.classification().era_id = "era_default";
-
-	state_.set_carryover_sides_start(config{"next_scenario", "tutorial"});
-}
-
 void game_launcher::mark_completed_campaigns(std::vector<config>& campaigns)
 {
 	for(config& campaign : campaigns) {

src/game_launcher.hpp

@@ -89,7 +89,6 @@ class game_launcher
 	bool is_loading() const;
 	void clear_loaded_game();
 	bool load_game();
-	void set_tutorial();
 	void set_test(const std::string& id);
 
 	/** Return the ID of the campaign to jump to (skipping the main menu). */

src/gui/dialogs/title_screen.cpp

@@ -315,14 +315,6 @@ void title_screen::pre_show(window& win)
 	//
 	register_button(win, "about", hotkey::HOTKEY_NULL, std::bind(&game_version::display<>));
 
-	//
-	// Tutorial
-	//
-	register_button(win, "tutorial", hotkey::TITLE_SCREEN__TUTORIAL, [this, &win]() {
-		game_.set_tutorial();
-		win.set_retval(LAUNCH_GAME);
-	});
-
 	//
 	// Campaign
 	//