Remove remnants of portsnap(8)

This was prompted by noticing that '/var/db/portsnap' still exists on
newly-installed machines.

With this change, all mentions of portsnap(8) in the tree are gone,
except for the historical note in the AUTHORS section of manpage
phttpget(8).

locate(1) will thus start indexing again '/var/db/portsnap' on machines
where this directory still exists, which may be a good way to push
administrators to delete it.

Reviewed by: cperciva
Approved by: emaste (mentor)
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45023