net ipv6: Remove unneded registration of an empty net/ipv6/neigh

sysctl no longer requires explicit creation of directories.  The neigh
directory is always populated with at least a default entry so this
should cause no user visible changes.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Acked-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: David S. Miller <davem@davemloft.net>