Always install backward compatibility timezones, as they are installed
on all major Linux distributions as well as NetBSD and OpenBSD.

Remove the undocumented ZONEINFO_OLD_TIMEZONES_SUPPORT and the deprecated
OLDTIMEZONES knobs as they are now the default.

Reviewed by: ngie, rgrimes
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D24306