Contacts marked as “Do Not Email” should not be deleted, because “Do Not Email” contacts also include people that clicked the unsubscribe link and chose do not email.
Deleting a contact may cause CAN-SPAM compliance issues. For example, if a “Do Not Email” contact is deleted, and then re-imported into a list, the contact will be sent future emails even though the contact chose to opt-out.
Keep in mind, CAN-SPAM requires contacts be able to unsubscribe for at least 30 days, so deleting contacts (or lists) before 30 days may violate CAN-SPAM. Per the FTC.gov website:
"Any opt-out mechanism you offer must be able to process opt-out requests for at least 30 days after you send your message."