Drop unused index index_namespaces_on_type_and_visibility_and_parent_id

What does this MR do and why?

Relates to Investigate dropping index_namespaces_on_type_a... (#425981 - closed), as part of Reduce LockManager LWLock Contention (&11543 - closed)

Drops the index index_namespaces_on_type_and_visibility_and_parent_id on namespaces, which is unused on gitlab.com.

We can see that the index has seen zero scans in the past month on any database:

source

and I confirmed that the feature the index was created to support is no longer used: #425981 (comment 1577595717)

MR acceptance checklist

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