Fix a dead store.

We would fail to clear DNS search list configuration if a router
stopped specifying the DNSSL RA option. I suspect that the bug
was mostly harmless, as the RDNSS and DNSSL options are typically used
together and omitting the RDNSS option would have the same effect.

CID: 1006219
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation