Remove bogus code to apply PathTargets to partial paths.

The partial paths that get modified may already have been used as
part of a GatherPath which appears in the path list, so modifying
them is not a good idea at this stage - especially because this
code has no check that the PathTarget is in fact parallel-safe.

When partial aggregation is being performed, this is actually
harmless because we'll end up replacing the pathtargets here with
the correct ones within create_grouping_paths().  But if we've got
a query tree containing only scan/join operations then this can
result in incorrectly pushing down parallel-restricted target
list entries.  If those are, for example, references to subqueries,
that can crash the server; but it's wrong in any event.

Amit Kapila

src/backend/optimizer/plan/planner.c

@@ -1758,19 +1758,6 @@ grouping_planner(PlannerInfo *root, bool inheritance_update,
 			}
 		}
 
-		/*
-		 * Likewise for any partial paths, although this case is simpler, since
-		 * we don't track the cheapest path.
-		 */
-		foreach(lc, current_rel->partial_pathlist)
-		{
-			Path	   *subpath = (Path *) lfirst(lc);
-
-			Assert(subpath->param_info == NULL);
-			lfirst(lc) = apply_projection_to_path(root, current_rel,
-											subpath, scanjoin_target);
-		}
-
 		/*
 		 * Save the various upper-rel PathTargets we just computed into
 		 * root->upper_targets[].  The core code doesn't use this, but it