ftp-proxy: Revert incorrect migration to libpfctl

libpfctl supports creating rules, but not (yet) adding addresses to a
pool. Adding addresses certainly does not work through adding a rule.

PR: 256917
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 8923ea6c867fd75b08b76883ec122c429a4018f9)