[l3vpn_ecmp] Fib flows are not deleted after extra route deletion

When the extra routes associated with a router are updated, the FIB flows corresponding to the old extra routes are not deleted.

After associating a VPN to the router, an extra router was associated to the router. The corresponding loadbalancing group and FIB flows were programmed correctly.

Then, the extra routes associated with the router is updated by using "router update" command. Now, the flows & groups of the new extra routes are also programmed correctly in the DPN.

But the the flows & groups of the removed extra route is still present in the DPN.