lib: remove vrf-interface config when removing the VRF

lib: remove vrf-interface config when removing the VRF

If we have the following configuration:
```
vrf red
 smth
 exit-vrf
!
interface red vrf red
 smth
```
And we delete the VRF using "no vrf red" command, we end up with:
```
interface red
 smth
```
Interface config is preserved but moved to the default VRF.

This is not an expected behavior. We should remove the interface config
when the VRF is deleted.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>