delete range of drop index for partition table is not correct.

[wjhuang2016]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

mysql> create table t (id int, c1 int, c2 int, primary key(id), key i1(c1), key i2(c2)) partition by range(id) (partition p0 values less than (6), partition p01 values less than (10) , partition p1 values less than maxvalue);
Query OK, 0 rows affected (0.01 sec)

mysql> alter table t  drop index i1;
Query OK, 0 rows affected (0.02 sec)

mysql> select * from mysql.gc_delete_range_done;
+--------+------------+----------------------------------------+----------------------------------------+--------------------+
| job_id | element_id | start_key                              | end_key                                | ts                 |
+--------+------------+----------------------------------------+----------------------------------------+--------------------+
|    147 |          2 | 74800000000000008f5f698000000000000002 | 74800000000000008f5f698000000000000003 | 432199516397240321 |
+--------+------------+----------------------------------------+----------------------------------------+--------------------+
1 row in set (0.00 sec)

2. What did you expect to see? (Required)

3 rows in gc_delete_range_done

3. What did you see instead (Required)

only 1 row.

4. What is your TiDB version? (Required)

master