Delete all option on subscribers tab

One way that we use mailing lists is for infrequent updates to a private list of subscribers that is managed outside of mlm For example, we have a mailing list of conference attendees that is collected (via opt-in) from that (non-Drupal) registration system. It's several hundred people, and changes in the weeks leading up to the event, but we send updates along the way.

We can set up a mailing list for it (hidden from public signup via CSS), but it would be tedious to clear out the subscriber list each time we want to reload a newer version. Within the same year the list is mostly additive, but we need to flush it completely for the next conference. The UI currently supports batch address import, it would be great to have a single click 'remove all subscribers' option without having to remove the list.