Discards a card if it cannot be played at the end of the turn.
This will discard cards that we know enough about to disqualify
them from being playable. For example, a card with an unknown
suit but a rank of 1 will not be playable if all the stacks have
been started. This rule also considers cards that can not be
played because their pre-requisite cards have already been discarded.
Description of the Osawa Discard action from Evaluating and Modelling Hanabi-Playing Agents.pdf: