fixtures: remove pointer receiver from NamedFixtures and allow purging tables which don't exist (yet) in ddb

When purging a table in ddb, we delete and re-create it. However, this currently fails if the table doesn't exist yet. As deleting a non-existing table achieves our goal (of the table no longer existing), we don't need to treat this as an error.

Additionally, removing the pointer receiver from fixtures.NamedFixtures allows for easier usage of the type without impacting performance too much (if at all).