To delete all contacts, simply go to Contacts, then All Contacts, and click on the three dots to the right of the Import Contacts button.
This will reveal two options:
1. Delete subscribed contacts - this will delete all contacts who are currently in a Subscribed status (i.e. able to receive messages).
2. Delete unsubscribed contacts - this will delete all contacts in an Unsubscribed state (i.e. not able to receive messages - a.k.a "Blacklisted" contacts)
NOTE - this action cannot be undone, so use it carefully!