ospf6d: Remove ospf6->external_id_table

ospf6d: Remove ospf6->external_id_table

The external_id_table was only ever used to store pointers to data
and was never used for lookup during the course of normal operations.
However it did lead to crashes because somewhere along the way
external routes stored in the external_table never had their
id associated into the external_id_table and we would assert
on the node lookup failing.

Since this code was never used for anything other than
storing data and it was never retrieved for anything useful
let's just remove it from ospf6d.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>