pf: Remove support for 'scrub fragment crop|drop-ovl'

The crop/drop-ovl fragment scrub modes are not very useful and likely to confuse
users into making poor choices.
It's also a fairly large amount of complex code, so just remove the support
altogether.

Users who have 'scrub fragment crop|drop-ovl' in their pf configuration will be
implicitly converted to 'scrub fragment reassemble'.

Reviewed by: gnn, eri
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D3466