Skip media handling when deleting cram/filter deck

This was part of pull request #208 but has been separated since it is less controversial and might be applied sooner alone. This change, upon deletion  of a dynamic deck, does not attempt to look for unused media that should be removed due to this deletion, as all cards of a dynamic deck continue to exist after the dynamic deck is deleted.

Beyond being a feature, this change bypasses a couple crash points for users, and does so in the most frequent source of deck deletions for many users.

For the record, I am not entirely sure of all possible decks that fall under the "dynamic" umbrella, but I do see in code where their cards are kept.