Revert "ripngd: Cleanup memory allocations on shutdown"

Revert "ripngd: Cleanup memory allocations on shutdown"

This reverts commit b1d29673ca16e558aea5d632da181555c83980cf.

This commit introduced a crash. When the VRF is deleted, the RIPNG
instance should not be freed, because the NB infrastructure still stores
the pointer to it. The instance should be deleted only when it's actually
deleted from the configuration.

To reproduce the crash:
```
frr# conf t
frr(config)# vrf vrf1
frr(config-vrf)# exit
frr(config)# router ripng vrf vrf1
frr(config-router)# exit
frr(config)# no vrf vrf1
frr(config)# no router ripng vrf vrf1
vtysh: error reading from ripngd: Resource temporarily unavailable (11)Warning: closing connection to ripngd because of an I/O error!
frr(config)#
```

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