ip6: Remove support for RFC2675 (Jumbo Payload Option)

The Jumbo Payload option was intended to allow the deployment of IPv6 on
networks with a link MTU in excess of 65,735 octets.

Speaking to one of the authors of RFC2675 the networks which motivated
the Jumbo Payload option no longer exist.

FreeBSD does not currently support any links with this capacity and
discussion when this change was first proposed suggested that the loop
back interface had to be patched to test implementation.

As there are no known devices that can carry Jumbo Payloads remove
support.

Reviewed by: glebius, teuxen, kp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D19960