Don't add specific id's to the readers fixtures

- This causes failures in the `test_create_resets_cached_counters` from
  the has_many_associations tests because sometimes a Post record gets
  created with id as 1 so the readers records get associated with it and
  `person.readers` returns these 2 records instead of empty array.
- Better to just remove the ids.

Author: prathamesh-sonpatki

activerecord/test/fixtures/readers.yml

@@ -1,11 +1,9 @@
 michael_welcome:
-  id: 1
   post_id: 1
   person_id: 1
   first_post_id: 2
 
 michael_authorless:
-  id: 2
   post_id: 3
   person_id: 1
   first_post_id: 3