[l3vpn_ecmp] Deleting MPLS GRE Tunnel port doesn't delete the bucket entry in the group

When MPLS GRE tunnel port is deleted the FIB entries on the OVS gets updated
whereas the group which has two bucket entries pointing to tunnel port is not updated

Expected Behavior
=================
When the MPLS GRE Tunnel port is deleted one of the bucket entry pointing to the tunnel port inside the Group should get deleted