Prepare to drop unused jira indexes on merge_requests

What does this MR do and why?

This merge request prepares two unused indexes to be removed from merge_requests.

See:

• #402497 (comment 1331759522)
• #403327 (closed)

Verification

Following the verification steps here:

select * from postgres_async_indexes where name LIKE 'index_merge_requests_on_target_project_id%';

 id |          created_at           |          updated_at           |                              name                              |                                        definition                                        |   table_name   | attempts | last_error
----+-------------------------------+-------------------------------+----------------------------------------------------------------+------------------------------------------------------------------------------------------+----------------+----------+------------
 78 | 2023-03-30 22:08:16.169006+00 | 2023-03-30 22:08:16.169006+00 | index_merge_requests_on_target_project_id_and_iid_jira_title   | DROP INDEX CONCURRENTLY "index_merge_requests_on_target_project_id_and_iid_jira_title"   | merge_requests |        0 |
 79 | 2023-03-30 22:08:16.188061+00 | 2023-03-30 22:08:16.188061+00 | index_merge_requests_on_target_project_id_iid_jira_description | DROP INDEX CONCURRENTLY "index_merge_requests_on_target_project_id_iid_jira_description" | merge_requests |        0 |
(2 rows)

bundle exec rails gitlab:db:reindex

gitlabhq_development=# gitlabhq_development=# \d 'index_merge_requests_on_target_project_id_and_iid_jira_title';
Did not find any relation named "index_merge_requests_on_target_project_id_and_iid_jira_title".
gitlabhq_development=# \d 'index_merge_requests_on_target_project_id_iid_jira_description';
Did not find any relation named "index_merge_requests_on_target_project_id_iid_jira_description".

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #403327 (closed)