Remove unused index on project_mirror_data

What does this MR do?

Removes an unused index - index_project_mirror_data_on_jid

We currently update the jid column on records in this table about 100 times a second, which is roughly 7% of total database time on the primary, and is our 4th most time consuming (writing) query.

Removing this index should improve this significantly as we will no longer have to update any indexes with each update.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content