Drop unused projects indexes for Gitlab.com

What does this MR do and why?

Drops unused projects indexes from Gitlab.com instance, it's done as part of Reduce LockManager LWLock Contention (&11543 - closed).

These are dropped only for Gitlab.com as it's used from ServicePing usage metrics which runs only in self-managed instances. Confirmation of this can be found in the corresponding issues.

• index_service_desk_enabled_projects_on_id_creator_id_created_at
• index_projects_on_mirror_id_where_mirror_and_trigger_builds
• index_projects_on_mirror_creator_id_created_at
• index_projects_on_id_service_desk_enabled
• idx_projects_id_created_at_disable_overriding_approvers_true
• idx_projects_id_created_at_disable_overriding_approvers_false

Closes: #428843 (closed), #428833 (closed), #428832 (closed), #428831 (closed), #428824 (closed) and #428817 (closed)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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