Revert "Move PartitionPruneInfo out of plan nodes into PlannedStmt"

Revert "Move PartitionPruneInfo out of plan nodes into PlannedStmt"

This reverts commit ec386948948c and its fixup 589bb816499e.

This change was intended to support query planning avoiding acquisition
of locks on partitions that were going to be pruned; however, the
overall project took a different direction at [1] and this bit is no
longer needed.  Put things back the way they were as agreed in [2], to
avoid unnecessary complexity.

Discussion: [1] https://postgr.es/m/4191508.1674157166@sss.pgh.pa.us
Discussion: [2] https://postgr.es/m/20230502175409.kcoirxczpdha26wt@alvherre.pgsql