pf: remove pf_remove_fragment()

Instead of having two functions pf_free_fragment() and pf_remove_fragment()
doing more or less the same, merge them into one. Just remove
fragment entries from the queue in pf_join_fragment() before they
are freed. Then pf_remove_fragment() is not needed anymore.
ok henning@

Obtained from: OpenBSD, bluhm <bluhm@openbsd.org>, dc8c96a470
Sponsored by: Rubicon Communications, LLC ("Netgate")