TaskQueueDB: no need for `TaskIndex` indexes, they can be dropped altogether.

The TQId,Value pair is the PRIMARY KEY in this table. So, the corresponding index is anyway created.
It is probably just better to drop the "TaskIndex": ["TQId"]

As additional context, this is what I see in the certification setup:

```
Dirac@dbod-dcertif.cern.ch
(MySQL):TaskQueueDB>describe `tq_TQToBannedSites`
+-------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| TQId | int unsigned | NO | PRI | | |
| Value | varchar(64) | NO | PRI | | |
+-------+--------------+------+-----+---------+-------+

2 rows in set
Time: 0.011s
Dirac@dbod-dcertif.cern.ch
(MySQL):TaskQueueDB>show INDEX FROM `tq_TQToBannedSites`
+--------------------+------------+------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+--------------------+------------+------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| tq_TQToBannedSites | 0 | PRIMARY | 1 | TQId | A | 0 | | | | BTREE | | | YES | |
| tq_TQToBannedSites | 0 | PRIMARY | 2 | Value | A | 0 | | | | BTREE | | | YES | |
| tq_TQToBannedSites | 1 | TaskIndex | 1 | TQId | A | 0 | | | | BTREE | | | YES | |
| tq_TQToBannedSites | 1 | BannedSitesIndex | 1 | Value | A | 0 | | | | BTREE | | | YES | |
+--------------------+------------+------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+

```

_Originally posted by @fstagni in https://github.com/DIRACGrid/DIRAC/pull/8462#discussion_r2845456126_