remove pointless line of code

If _size_menu was non-null, we just return it. Ergo, it was already null when we reached
this line, which makes calling delete on it completely pointless

gtk2_ardour/time_axis_view.cc

@@ -1362,8 +1362,6 @@ TimeAxisView::build_size_menu ()
 		return;
 	}
 
-	delete _size_menu;
-
 	using namespace Menu_Helpers;
 
 	_size_menu = new Menu;