zebra: remove unnecessary check for default vrf

zebra: remove unnecessary check for default vrf

The default vrf is generally non-NULL, except when shutdown. So, most
of the time it is not necessary to check if it is NULL, we should
remove the useless checks for it.

Searched them with exact match:
```
grep -rI "zebra_vrf_lookup_by_id(VRF_DEFAULT)" | wc -l
31
```

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