zserv: simplify handling of route delete requests

zserv: simplify handling of route delete requests

Route attributes like tag, distance and metric are irrelevant when we
want to delete a route from a client daemon. The same can be said about
the nexthops of the route. Only the IP prefix and client protocol are
enough to identify the route we want to remove, considering that zebra
maintains at most one route from each client daemon for each prefix. Once
rib_delete() is called, it deletes the selected route with all of its
nexthops.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>