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