Relax query flattener constraint (3b) and thereby allow flattening the RHS of

Relax query flattener constraint (3b) and thereby allow flattening the RHS of
a LEFT JOIN even if the RHS contains a virtual table.  This was previously
disallowed by [9dbae1df75219e2a] as a performance optimization.  It
turns out that the constraint causes performance issues, and we do not have
a record of any performance issue that it solves.

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