In virNWFilterObjLoad we can still fail after virNWFilterObjAssignDef,
but we don't unlock and free the created virNWFilterObjPtr in the
cleanup path.

The bit we are trying to do after AssignDef is just STRDUP in the
configFile path. However caching the configFile in the NWFilterObj
is largely redundant and doesn't follow the same pattern we use
for domain and network objects.

So just remove all the configFile caching which fixes the latent
bug as a side effect.