Remove unused struct in6_ndifreq; dead since 2013 sys/netinet6/in6_var.h r1.37

Remove unused struct in6_ndifreq;  dead since 2013 sys/netinet6/in6_var.h r1.37

    Remove unused code manipulating a default interface and its index
    This is a leftover from the on-link assumption behavior removal,
    which has been deprecated by RFC4861 anyway.

Outside of _KERNEL, but nothing in base uses it, either.
codesearch.debian.net seems to agree.

OK mvs claudio bluhm