zebra: remove kernel_delete_ipv6_old()

The only user of this was rib_bogus_ipv6(), which was removed in the
previous commit.  Good riddance.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
Acked-by: Greg Troxel <gdt@ir.bbn.com>
Acked-by: Feng Lu <lu.feng@6wind.com>
Acked-by: Paul Jakma <paul@jakma.org>
5 files changed