While working on SERVER-41746 I noticed that it was impossible to insert collection names with a length of >= ~8MB. When a new entry is added into the DurableCatalog, we append the 'ns' field twice here.

{

    ns: <String>,

    ...

    md: {

        ns: <String>,

        ...

    },

    ...

}

With a collection name that is <= ~8MB, that quickly brings us up to our 16MB BSON document limit.
Removing one of these fields would lower the cost of having to maintain both the fields whenever a namespace change occurred and allows the insertion of even longer collection names.