Final removal of WITH_NEW_XORG

This knob was turned on for all version of FreeBSD in r369875
(2014-10-03) and officially deprecated. Since then, it was gradually
removed from many ports when they needed an update.

x11-servers/xorg-server was the last major user of this knob. The port
was updated to xserver 1.14 in r374982 (2014-12-19). The update got rid
of the knob, clearing the path to the final removal.

This commit changes ports who were checking for WITH_NEW_XORG and remove
its handling from bsd.port.mk.

While here, two sanity checks are added to warn user about WITH_KMS and
WITH_NEW_XORG which have no effect now.

Differential Revision: https://reviews.freebsd.org/D1351
Reviewed by: antoine, bapt, kwm
Approved by: portmgr (antoine, bapt), kwm