Drop factory linting

Problem

Lint factories (as described in https://github.com/thoughtbot/factory_bot/blob/master/GETTING_STARTED.md#linting-factories) is slow and has caused some issues in the past:

• #436420 (closed)
• #478114 (closed)
• #474262 (closed)
• gitlab-org/quality/engineering-productivity/master-broken-incidents#7963 (closed)

Proposed solution

Although lint factories can be helpful we should consider dropping it and rely on other specs (which use these factories) to catch issues related to factory setup.

Pros

• Faster CI (we spend ~48 minutes linting factories - see https://gitlab-org.gitlab.io/rspec_profiling_stats/)
• Less flaky specs
• Less magical code to maintain

Cons

• Invalid factories are only caught in other specs