Fix spec following factory_bot changes

In factory_bot 5.0.0, when the 'build' strategy is used, the
associations are also built. Previous to 5.0.0, they were created.

See the changelog for details:
https://github.com/thoughtbot/factory_bot/blob/0a072064c536818bec517cd00b56ac36b09c0f4a/NEWS#L3-L4

Also include minor changes to remove unneeded variables