6lowpan: do garbage collect unreachable NCEs

RFC6775 says "if NUD on the router determines that the host is UNREACHABLE (based on the logic in [RFC4861]), the NCE SHOULD NOT be deleted but rather retained until the Registration Lifetime expires." Since this is a "SHOULD NOT" and not a "MUST NOT", we can still remove these NCEs to circumvent NCEs going into UNREACHABLE and never leave this state again. Actually removing these entries from the cache seems also be the better choice for memory constraint device.