Investigate dropping index_namespaces_on_type_and_visibility_and_parent_id

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

• Find out when and why the index was added
• Decide if the index can be dropped
• Drop the index if possible - !132688 (merged)
• If the index cannot be dropped, document the reason in a comment on this issue and close.