Cells: Fix cross joins in backup related code

What does this MR do and why?

Refactor: Removes unnecessary cross-table-joins which will not be allowed in the future (Cells) when namespaces and users tables are stored in separate databases.

Resolves #417467 (closed)

Why is it safe?

See #417467 (comment 1514678828):

Looks like these namespaces-users joins were added in !40679 (7ffa9e59).

At the time, it was stated that the change was made to avoid N+1 queries. Note that the N+1 that the N+1 test was checking for was the one that was fixed by including route. That's the only reason the N+1 test exists. It was also stated that we don't expect the owner N+1s to be a problem in practice: !40679 (comment 409374466).

That's to say, I'm not concerned that this will cause a performance regression. Iterating over ProjectWiki is not the common usage.

Additionally, backups:

• are performed completely outside of web requests and Sidekiq jobs
• iterate over potentially all projects
• are expected to take a long time in many cases

Therefore I expect that we can remove the users table (the owner association) from the includes calls, and add an exception for any auto-detected N+1s.

Furthermore, it turns out that the N+1 test doesn't fail on this change.

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