The `renew_lease` method is entirely unused and will likely remain so for a long time

There are no callers of renew_lease in Tahoe-LAFS. As of #220 (closed) there are no callers in ZKAPAuthorizer, either. Tahoe-LAFS 1.16 is also dropping this method from the interface.

Thus, we can drop this code from ZKAPAuthorizer.