Fix cross joins in merge_requests_finder

What does this MR do and why?

Fixes cross joins in merge_requests_finder by removing the line that allows cross joins across databases.

Part of the Cells: Fix all the cross joins between users an... (&10973 - closed) epic.

MR acceptance checklist

• I have evaluated the MR acceptance checklist for this MR.

Related to #419220 (closed)

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