remove old perl entries from ObsoleteFiles.inc

Each entry in ObsoleteFiles.inc adds to the time `make delete-old` and
friends take to run.  Perl was removed from the FreeBSD base system a
very long time ago (FreeBSD 5); source updates have not been supported
from that version for years.

Perl was a single component responsible for thousands of entries so
provides significant benefit with little effort/investigation required.
We could still use a more comprehensive cleanup to remove old entries.

Also add an UPDATING note (with wordsmithing by imp) indicating that
`make delete-old` is required along each step of a source upgrade from
an old, unsupported release.

Discussed with:	imp
Sponsored by:	The FreeBSD Foundation

0 files changed, 0 insertions, 0 deletions