GUI2: removed some unnecessary calls to canvas::set_is_dirty

The one in Outro was unnecessary since set_variable already calls set_is_dirty.
The one in the Story Viewer was there to deal with some background redraw issues which no longer appear
with the new drawing methods.

src/gui/dialogs/outro.cpp

@@ -96,7 +96,6 @@ void outro::draw_callback(window& window)
 	canvas& window_canvas = window.get_canvas(0);
 
 	window_canvas.set_variable("fade_step", wfl::variant(fade_step_));
-	window_canvas.set_is_dirty(true);
 
 	if(fading_in_) {
 		fade_step_ ++;

src/gui/dialogs/story_viewer.cpp

@@ -217,9 +217,6 @@ void story_viewer::display_part(window& window)
 
 	window_canvas.set_cfg(cfg);
 
-	// Needed to make the background redraw correctly.
-	window_canvas.set_is_dirty(true);
-
 	//
 	// Title
 	//