You probably had Time Pendulum assigned by the Oracle as your card of the day. You cannot remove those cards. Each day the Oracle will assign you a new card; you can choose to build a deck based on it, or you can keep your current one (although it will only keep playing if you are ranked above 500 on your level's leaderboard.) Wait until tomorrow and see what you get, or better yet, try to work with the card to improve your overall game skills. Arena deck building can be good practice for learning how other decks and styles work.
