Previously we had two `config.around(:each, :migration)` blocks that
disabled and re-enabled transactional tests. However, the
`delete_from_all_tables!` call made in the outer block may initiate a
transaction as a result.

To avoid this and clear up confusion, we consolidate
`delete_from_all_tables!` in one place.

I attempted to do something similar in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/101613 but
reverted this change in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/101824 due to
issues with schema versions not being consistent.
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/101824 likely
solved the root issue by forcing a refresh of attribute methods.