proposal: remove "success" and "error" Button statuses

[giogonzo]

As per recent changes to the guidelines, we never use display feedback on the action performed directly inside the button UI (we typically display it alongside the submit area using different types of error UI)

For this reason, showing feedback should not be handled by this component anymore
(while the "loading" state should remain)

[giogonzo]

Discussed offline with the rest of the team, the perception is that it would be too speculative as a change right now. Also, we might want to remove the "error" case only and keep the "success" one instead.