bgpd: withdraw fib entry on appropriate table identifier

bgpd: withdraw fib entry on appropriate table identifier

There are cases where the table identifier is set on a bgp entry, mainly
due to route-map, and associate fib entry needs to be removed.
This change encompasses also the route-map reconfiguration that leads to
removing the previous entry, whereas bgp update had been triggered (
this happens when software inbound reconfiguration is handled).

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>