*: remove ZEBRA_INTERFACE_VRF_UPDATE

*: remove ZEBRA_INTERFACE_VRF_UPDATE

Currently when one interface changes its VRF, zebra will send these messages to
all daemons in *order*:
    1) `ZEBRA_INTERFACE_DELETE` ( notify them delete from old VRF )
    2) `ZEBRA_INTERFACE_VRF_UPDATE` ( notify them move from old to new VRF )
    3) `ZEBRA_INTERFACE_ADD` ( notify them added into new VRF )

When daemons deal with `VRF_UPDATE`, they use
`zebra_interface_vrf_update_read()->if_lookup_by_name()`
to check the interface exist or not in old VRF. This check will always return
*NULL* because `DELETE` ( deleted from old VRF ) is already done, so can't
find this interface in old VRF.

Send `VRF_UPDATE` is redundant and unuseful. `DELETE` and `ADD` are enough,
they will deal with RB tree, so don't send this `VRF_UPDATE` message when
vrf changes.

Since all daemons have good mechanism to deal with changing vrf, and don't
use this `VRF_UPDATE` mechanism.  So, it is safe to completely remove
all the code with `VRF_UPDATE`.

Signed-off-by: anlan_cs <anlan_cs@tom.com>