IPoIB: Fix build now that neighbour destructor is in neigh_params

Fix the IPoIB build (which is broken in net-2.6.17 because of my
screw-up, which left out this chunk in ipoib_multicast.c). 
The neighbour destructor is now in neigh_params, so we don't
need to clear it in the ops structure.

Signed-off-by: Roland Dreier <rolandd@cisco.com>