Remove diagnostic query ListXssPartialPaths

[jeongsoolee09]

What This PR Contributes

This PR removes the diagnostic query ListXssPartialPaths. This decision is made based on the following objections:

1. The partial paths are subject to change as the underlying data flow library evolves. This means the test breakage can come from the standard library and not our code.
2. If the model-as-data summaryModel predicate needs to be tested, this query can be trimmed down to target the predicate directly (or in a more reader-friendly way, using ModelsAsData::SummarizedCallableFromModel.getACall/0 or summaryStep / summaryStepNodes from the same module).
3. The query depends on the outdated data flow APIs.

Future Works

N/A

[data-douser]

@jeongsoolee09 Regarding your second objection (from PR description), does removing this diagnostics query in any way leave a new (or increased) gap in our testing of the models-as-data summaryModel predicate? If not, is there some form of test for summaryModel that we might want to improve in the future (and track now via a new issue)?

[data-douser]

Pending resolution of one comment / question. Other than that, LGTM!

[jeongsoolee09]

@data-douser No, it's closer to a supposed scenario where summaryModel ever needs to be tested. It is currently not tested anywhere in the codebase.