`cascade-zone remove` leaves `state.db` (and key files) on file system

[jpmens]

A zone's state remains on the file system when a zone is removed:

$ cascade zone remove e1.net
Removed zone e1.net

$ ls -l zone-state/e1.net.db 
-rw------- 1 jpm jpm 6222 Nov 28 12:33 zone-state/e1.net.db

I also note, but this might be on purpose so as to re-use keys, that the key files and their configuration also remain in place.