zebra: fix missing vni transition

zebra: fix missing vni transition

`show evpn vni detail` doesn't reflect any change in vni transition.
Need to add processing in command of `[no] vni (1-16777215)`.

With the config:
```
!
vni 66
!
vrf vrf1
 vni 88
 exit-vrf
!
```

Before:
```
(config-vrf)# no vni 88
(config-vrf)# do show evpn vni detail
VNI: 66
  Type: L3
  Tenant VRF: default
  L2 VNIs: <- Empty
```

After:
```
(config-vrf)# no vni 88
(config-vrf)# do show evpn vni detail
VNI: 66
  Type: L3
  Tenant VRF: default
  L2 VNIs: 88 <-
```

Signed-off-by: anlan_cs <vic.lan@pica8.com>