Fix double free, heap-use-after-free

_canvas_grid_zone and _section_box are destroyed when the
Canvas Container (here: `_track_canvas_viewport`) deletes
child items.

gtk2_ardour/editor.cc

@@ -917,12 +917,10 @@ Editor::~Editor()
 	delete _snapshots;
 	delete _sections;
 	delete _locations;
-	delete _canvas_grid_zone;
 	delete _properties_box;
 	delete selection;
 	delete cut_buffer;
 	delete _cursors;
-	delete _section_box;
 
 	LuaInstance::destroy_instance ();