net/miniupnpd: remove ipfw port option

It is well-known that net/miniupnpd does not work with IPFW. However,
the port options still allow building with IPFW support. In order to
prevent unnecessary foot-shooting, let us eliminate this option outright
and unconditionally use pf instead.

PR: 271732
Approved by: squat@squat.no (maintainer timeout, two weeks)
Sponsored by: Rubicon Communications, LLC ("Netgate")