Drop path_hash

**DO NOT MERGE UNTIL https://github.com/alphagov/bouncer/pull/107 merged, otherwise Bouncer's tests will fail**

By now (if https://github.com/alphagov/bouncer/pull/107 is merged) nothing is using it, not here or in Bouncer or in any part of the
DB (was being used, albeit harmlessly in `index_mappings_on_site_id_and_path_hash`)