Drop unused partitioned_foreign_keys table

What does this MR do and why?

Related issue: #335046 (closed)

Drop the partitioned_foreign_keys table. This table was added as part of a feature to support foreign keys referencing partitioned tables, before native support was available in postgres.

However, this feature was never used. Once we upgraded to PG12 in %14.0 it was no longer needed, as support was added in postgres. We removed the code in !64658 (merged) in %14.1

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