Investigate dropping index_projects_on_mirror_id_where_mirror_and_trigger_builds

As part of reducing lightweight lock contention, investigate if it's possible to drop the index index_projects_on_mirror_id_where_mirror_and_trigger_builds from the projects table

• Find out when and why the index was added
• Decide if the index can be dropped
• Drop the index if possible
• If the index cannot be dropped, document the reason in a comment on this issue and close.