Some of the cards do not implement the erase method that each card
should have. However, having an empty method in each of those classes
does not make too much sense. Lets rather have an erase method in the
superclass (Card) that prints a warning to inform the user that erasing
the spcified card is not supported.
Change-Id: If5add960ec0cab58a01d8f83e6af8cb86ec70a8d