base: remove if_wg(4) and associated utilities, manpage

base: remove if_wg(4) and associated utilities, manpage

After length decisions, we've decided that the if_wg(4) driver and
related work is not yet ready to live in the tree.  This driver has
larger security implications than many, and thus will be held to
more scrutiny than other drivers.

Please also see the related message sent to the freebsd-hackers@
and freebsd-arch@ lists by Kyle Evans <kevans@FreeBSD.org> on
2021/03/16, with the subject line "Removing WireGuard Support From Base"
for additional context.