zebra: Only send down pertinent information on RTM_DELROUTE

zebra: Only send down pertinent information on RTM_DELROUTE

Background:

v6 does not have route replace semantics.  If you want to add a nexthop
to an existing route, you just send RTM_NEWROUTE and the new nexthop.
If you want to delete a nexthop you should just send RTM_DELROUTE
with the removed nexthop.

This leads to situations where if zebra is processing a route
and has lost track of intermediate nexthops( yes this sucks )
then v6 routes will get out of sync when we try to implement
route replace semantics.

So notice when we are doing a route delete and the route is
not being updated, just send the prefix and tell it too delete.

Ticket: CM-20391
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>