some adjacent limit is redundant and unnecessary

[AilinKid]

Enhancement

drop table if exists t;
create table t(a int primary key, b int);
insert into t values (1, 11), (4, 44), (2, 22), (3, 33);
set session tidb_executor_concurrency = 4;
set @@session.tidb_hash_join_concurrency = 5;
set @@session.tidb_distsql_scan_concurrency = 15;

mysql> explain select a from (select a from t where b > 2 order by a limit 3 offset 1) as t1 order by a limit 2 offset 1;
+--------------------------------+---------+-----------+---------------+-------------------------------+
| id                             | estRows | task      | access object | operator info                 |
+--------------------------------+---------+-----------+---------------+-------------------------------+
| Limit_18                       | 1.33    | root      |               | offset:1, count:2             |
| └─Limit_44                     | 1.33    | root      |               | offset:1, count:3             |
|   └─TableReader_49             | 1.33    | root      |               | data:Limit_48                 |
|     └─Limit_48                 | 1.33    | cop[tikv] |               | offset:0, count:4             |
|       └─Selection_29           | 1.33    | cop[tikv] |               | gt(test.t.b, 2)               |
|         └─TableFullScan_28     | 4.00    | cop[tikv] | table:t       | keep order:true, stats:pseudo |
+--------------------------------+---------+-----------+---------------+-------------------------------+
6 rows in set (0.00 sec)

expected
Limit_24	2.00	root		offset:2, count:2
  └─TableReader_32	4.00	root		data:Limit_33
    └─Limit_33	4.00	cop[tikv]		offset:0, count:4
      └─Selection_30	4.00	cop[tikv]		gt(planner__cascades__integration.t.b, 2)
        └─TableFullScan_31	4.00	cop[tikv]	table:t	keep order:true, stats:pseudo