ldpd: fix bug with dual-stack neighbors

ldpd: fix bug with dual-stack neighbors

We were assuming that a neighbor can be deleted only when all of its
adjacencies are dead. This is not the case for dual-stack neighbors. If
the transport-preference is IPv4 and all adjacencies are IPv6 (or
vice-versa), then it should be deleted and everything cleaned-up
accordingly.

Bug exposed by the new RB tree implementation on master, but the fix
also applies to stable/3.0.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>