bgp: T6189: explicitly call vtysh to remove VRF L3VNI configuration

After e7bb65894 ("vrf: T6189: render FRR L3VNI configuration when creating VRF
instance") we need to ensure that the VRF L3VNI configuration is removed in FRR
prior to removing the BGP VRF instance.

The reason is [1] where FRR only allows VRF BGP instance to be removed when
there is NO VNI configured anymore.

1: https://github.com/FRRouting/frr/blob/064c3494527b9e84260410006768ed38e57e1de7/bgpd/bgp_vty.c#L1646-L1650