Smart Search - sync mysql with postgresql database

[csthomas]

Summary of Changes

This changes has been designed for J4 at #14783, but it should be merged also for J3.7.

• Sync changes from [com_finder] - SQL change for removeOrphanNodes() and postgresql driver fix #12313 in smart search from postgresql to mysql.
• Add $query->clear() at the top of loop.

Testing Instructions

Go to backend - Smart Search extension.
Click on button "Index" - all should work as before or better.

Expected result

Works.

Documentation Changes Required

No.

[tonypartridge]

I have tested this item ✅ successfully on 47c0553

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[marcodings]

I have tested this item ✅ successfully on 47c0553

Tested against j37b3

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