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.