Drop index namespaces_on_ldap_sync_last_update_at

What does this MR do and why?

Removes the unused index index_namespaces_on_ldap_sync_last_update_at from namespaces.

source

More info on the decision to drop the index can be found here and here.

How to set up and validate locally

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

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 #425976 (closed)