Remove the SIOCSIFALIFETIME_IN6 ioctl.

The SIOCSIFALIFETIME_IN6 provided by the kame project is unused,
it can't really be used safely and has been completely removed from
NetBSD and OpenBSD.

Obtained from: NetBSD (kern/35897)
PR: 210148 (exp-run)
Reviewed by: ae, hrs
Relnotes: yes
Approved by: re (glebius)
Differential Revision: https://reviews.freebsd.org/D5491