pf: drop support for fragment crop|drop-ovl

We removed the code for these modes back in 2015, but converted such
configurations to 'scrub fragment reassemble'. It's been long enough,
drop the backwards compatibility glue too.

Reviewed by: mjg
MFC after: never
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D37460