`release_dependencies` has legacy project urls

[miketheman]

In the pypi/legacy, a kind = 8 existed. https://github.com/pypi/legacy/blob/a418131180a14768328713df5533c600c425760d/store.py#L76-L84

It was ported over to warehouse at inception with a TODO to extras and backfill to a separate table, which was completed in #11566

warehouse=> select count(*) from release_dependencies where kind = 8;
  count  
---------
 1174262

As the backfill job moved these to the release_urls table, they should no longer be needed, and could be removed once confirmed.