Remove dead code in get_param_path_clause_serials()

The function get_param_path_clause_serials() is used to get the set of
pushed-down clauses enforced within a parameterized Path.  Since we
don't currently support parameterized MergeAppend paths, and it
doesn't look like that is going to change anytime soon (as explained
in the comments for generate_orderedappend_paths), we don't need to
consider MergeAppendPath in this function.

This change won't make any measurable difference in performance; it's
just for clarity's sake.

Author: Richard Guo
Reviewed-by: Andrei Lepikhov
Discussion: https://postgr.es/m/CAMbWs4_Puie4DQ2ODvjQB_3CxYkUODnrJm8jn_ObMAcrjYNW7Q@mail.gmail.com

src/backend/optimizer/util/relnode.c

@@ -1911,6 +1911,13 @@ get_param_path_clause_serials(Path *path)
 {
 	if (path->param_info == NULL)
 		return NULL;			/* not parameterized */
+
+	/*
+	 * We don't currently support parameterized MergeAppend paths, as
+	 * explained in the comments for generate_orderedappend_paths.
+	 */
+	Assert(!IsA(path, MergeAppendPath));
+
 	if (IsA(path, NestPath) ||
 		IsA(path, MergePath) ||
 		IsA(path, HashPath))
@@ -1964,27 +1971,6 @@ get_param_path_clause_serials(Path *path)
 		}
 		return pserials;
 	}
-	else if (IsA(path, MergeAppendPath))
-	{
-		/* Same as AppendPath case */
-		MergeAppendPath *apath = (MergeAppendPath *) path;
-		Bitmapset  *pserials;
-		ListCell   *lc;
-
-		pserials = NULL;
-		foreach(lc, apath->subpaths)
-		{
-			Path	   *subpath = (Path *) lfirst(lc);
-			Bitmapset  *subserials;
-
-			subserials = get_param_path_clause_serials(subpath);
-			if (lc == list_head(apath->subpaths))
-				pserials = bms_copy(subserials);
-			else
-				pserials = bms_int_members(pserials, subserials);
-		}
-		return pserials;
-	}
 	else
 	{
 		/*