Remove open TODO in thumbnail.py

When running
:set thumbnail.filmstrip!
or
:set thumbnail.filmstrip false
it seems logical to hide the filmstrip when not in thumbnail mode.
Entering thumbnail mode seems a bit of a stretch as we never auto-enter
modes.

vimiv/gui/thumbnail.py

@@ -514,7 +514,6 @@ def _on_view_changed(self, filmstrip: bool):
         else:
             _logger.debug("Setting view mode to icon")
             self.setViewMode(self.IconMode)
-            # TODO the alternative is to enter thumbnail mode
             if api.modes.current() != api.modes.THUMBNAIL:
                 self.hide()
         self._update_geometry()