refactor: remove DeckDropDownAdapter

[david-allison]

Unused

Checklist

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner

[lukstbit]

This class, DeckDropDownAdapter, is unused and should be removed(my fault because I didn't see this when removing the toolbar deck selection Spinner).

The only thing keeping it alive is the inner SubtitleProvider interface which is used in CardBrowser but that usage can be removed by using the numberOfCardsOrNoteShown property directly instead of deckDropDownSubtitle from the interface.

[lukstbit]

Great!

Edit: Still more code to delete...

[BrayanDSO]

Deleting code 😍😍😍