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

(cherry picked from commit 9b30b96c1fa4ee0dfc540878fbb3247bf92d19eb)

ObsoleteFiles.inc: Remove /var/db/portsnap

This is a followup to commit "Remove remnants of portsnap(8)"
(9b30b96c1fa4). I wasn't aware of OLD_DIRS.

Approved by: markj (mentor)
MFC after: 1 day
MFC with: 9b30b96c1fa4
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45023

(cherry picked from commit 54bbcc09a72b5d7a581f51416c9e635a33c2f884)

Approved by: markj (mentor)