Be consistent with production: don't set synchronous_commit in dev

- This was used for Postgres in dev for some reasons, but isn't set in production, and we should have no disparity between production and dev anymore. Remove it from the test database config as well for good measure.
`load_infile` here and below should have been removed before - I think it's a mysql-specific option, and we don't have it [in production](https://github.gds/gds/alphagov-deployment/blob/master/transition/to_upload/database.yml) any more.