sysutils/etcupdate: Remove obsolete port.

etcupdate has been included in the base system since FreeBSD 10.0 and
this port was only useful for older OS versions where it was not
included in the base system.

Reported by: Jamie Landeg-Jones <jamie@catflap.org>
Reviewed by: fuz, matthew
Differential Revision: https://reviews.freebsd.org/D39188