Redundant DISTINCT in set-operation branches creates unnecessary temporary tables

## Description

MariaDB retains `DISTINCT` inside both branches of `UNION`, `INTERSECT`, and

`EXCEPT`, although the set operation already provides distinct set semantics.

### Expected behaviour

No branch-level temporary table should be required. The branch `DISTINCT`

operators should be removed or merged with the set operation.

### Actual behaviour

`EXPLAIN FORMAT=JSON` reports a `temporary_table` inside each branch in

addition to the `union_result`, `intersect_result`, or `except_result`.

| Operation | Without branch DISTINCT | With branch DISTINCT | Slowdown |

|---|---:|---:|---:|

| UNION | 62.57 ms | 84.94 ms | 1.36x |

| INTERSECT | 51.98 ms | 72.63 ms | 1.40x |

| EXCEPT | 44.83 ms | 68.46 ms | 1.53x |

## How to repeat

```sql

DROP DATABASE IF EXISTS mariadb_set_branch_distinct;

CREATE DATABASE mariadb_set_branch_distinct;

USE mariadb_set_branch_distinct;

CREATE TABLE digits(d INT PRIMARY KEY);

INSERT INTO digits VALUES(0),(1),(2),(3),(4),(5),(6),(7),(8),(9);

CREATE TABLE lhs(id INT, v INT);

CREATE TABLE rhs(id INT, v INT);

INSERT INTO lhs SELECT n,MOD(n,1000) FROM

 (SELECT a.d+b.d*10+c.d*100+d.d*1000+e.d*10000+1 n

  FROM digits a,digits b,digits c,digits d,digits e) numbers;

INSERT INTO rhs SELECT n,MOD(n,1000) FROM

 (SELECT a.d+b.d*10+c.d*100+d.d*1000+e.d*10000+50001 n

  FROM digits a,digits b,digits c,digits d,digits e) numbers;

ANALYZE TABLE lhs,rhs;

EXPLAIN FORMAT=JSON SELECT COUNT(*) FROM

 ((SELECT DISTINCT id FROM lhs) UNION (SELECT DISTINCT id FROM rhs)) s;

EXPLAIN FORMAT=JSON SELECT COUNT(*) FROM

 ((SELECT DISTINCT id FROM lhs) INTERSECT (SELECT DISTINCT id FROM rhs)) s;

EXPLAIN FORMAT=JSON SELECT COUNT(*) FROM

 ((SELECT DISTINCT id FROM lhs) EXCEPT (SELECT DISTINCT id FROM rhs)) s;

SELECT COUNT(*) FROM ((SELECT id FROM lhs) UNION (SELECT id FROM rhs)) s;

SELECT COUNT(*) FROM

 ((SELECT DISTINCT id FROM lhs) UNION (SELECT DISTINCT id FROM rhs)) s;

SELECT COUNT(*) FROM ((SELECT id FROM lhs) INTERSECT (SELECT id FROM rhs)) s;

SELECT COUNT(*) FROM

 ((SELECT DISTINCT id FROM lhs) INTERSECT (SELECT DISTINCT id FROM rhs)) s;

SELECT COUNT(*) FROM ((SELECT id FROM lhs) EXCEPT (SELECT id FROM rhs)) s;

SELECT COUNT(*) FROM

 ((SELECT DISTINCT id FROM lhs) EXCEPT (SELECT DISTINCT id FROM rhs)) s;

```