mysql: fix use of wrong SQL index when checking for dropped partitions

After partitions were dropped with TokuDB, we had a case where MariaDB 10.1.14
stopped using the most appropriate index.

MariaDB [neo0]> explain SELECT DISTINCT data_id FROM obj WHERE `partition`=5;
+------+-------------+-------+-------+-------------------+---------+---------+------+------+---------------------------------------+
| id   | select_type | table | type  | possible_keys     | key     | key_len | ref  | rows | Extra                                 |
+------+-------------+-------+-------+-------------------+---------+---------+------+------+---------------------------------------+
|    1 | SIMPLE      | obj   | range | PRIMARY,partition | data_id | 11      | NULL |   10 | Using where; Using index for group-by |
+------+-------------+-------+-------+-------------------+---------+---------+------+------+---------------------------------------+
MariaDB [neo0]> SELECT SQL_NO_CACHE DISTINCT data_id FROM obj WHERE `partition`=5;
Empty set (1 min 51.47 sec)

Expected:

MariaDB [neo1]> explain SELECT DISTINCT data_id FROM obj WHERE `partition`=4;
+------+-------------+-------+------+-------------------+---------+---------+-------+------+------------------------------+
| id   | select_type | table | type | possible_keys     | key     | key_len | ref   | rows | Extra                        |
+------+-------------+-------+------+-------------------+---------+---------+-------+------+------------------------------+
|    1 | SIMPLE      | obj   | ref  | PRIMARY,partition | PRIMARY | 2       | const |    1 | Using where; Using temporary |
+------+-------------+-------+------+-------------------+---------+---------+-------+------+------------------------------+
1 row in set (0.00 sec)
MariaDB [neo1]> SELECT SQL_NO_CACHE DISTINCT data_id FROM obj WHERE `partition`=4;
Empty set (0.00 sec)

Restarting the server or 'OPTIMIZE TABLE obj; ' does not help.

Such issue could prevent the cluster to start due to timeouts, by always going
back to RECOVERING state.
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