This removes the upgrade button as default content in CourseCardActionSlot.

This is, for the moment, a Sumac-specific change.  The button is still
present in master, pending proper removal (including the component
definitions from the rest of the repository) prior to the U release by
the maintainers.  See:

https://github.com/openedx/frontend-app-learner-dashboard/issues/438