Remove user from explore/groups queries

What does this MR do and why?

Describe in detail what your merge request does and why.

Should solve the performance issues on the /explore/groups endpoint on gitlab.com. Currently there's a massive speed difference between that page loaded anonymously and as a user, but as it supposedly just lists "public" projects anyway I don't see why there should be a difference, especially as it being slower makes it less usable in the first place.

Curiously this still passes the existing specs, so I haven't added made any changes to those 🤷 This is currently marked as an experiment in the feature flag as the performance issue only really shows up on .com and so I need to see its impact in production.

Related #358944 (closed)

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