nd6: Remove DRAFT_IETF_6MAN_IPV6ONLY_FLAG and EXPERIMENTAL options
The draft-ietf-6man-ipv6only-flag has been obsoleted by RFC 8925.
Remove the EXPERIMENTAL compile option from the kernel and remove
DRAFT_IETF_6MAN_IPV6ONLY_FLAG from userland.
This compile option was not enabled by default.
Also regenerate src.conf.5.
Reviewed by: bz
Differential Revision: https://reviews.freebsd.org/D56228