kern: wg: add support for removing Allowed-IPs

This was recently added to Linux to improve incremental update support,
as you could previously add Allowed-IPs but not remove without replacing
the whole set (and thus, potentially disrupting existing traffic).

Removal is incredibly straightforward; we'll find it in p_aips first
to ensure that it's actually valid for this peer, then we'll delete it
from the radix tree before we remove the corresponding p_aips entry.

Reviewed by: Jason A. Donenfeld, jhb
Differential Revision: https://reviews.freebsd.org/D50448