GH-4646 remove intermediate join variables

[JervenBolleman]

GitHub issue resolved: #4646

Briefly describe the changes proposed in this PR:

The PathIteration adds variables to the binding set that are not known ahead of time and not needed after the pathiteration logic is done. These should be stripped out and not propegated.

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• I've added tests for the changes I made
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[abrokenjester]

Looks good to me - great detective work!

[JervenBolleman]

I will merge it into main now, for 5.x I hope to make a better solution. That still needs work, and expanded test coverage.