Dropping a sharded collection doesn't get rid of the CSS entry

Currently, when a collection is dropped, we only clear the CSR's contents, but don't actually drop the CSS entry from the CSS map.

With the recent changes to fix the range deleter preservation on Shard Primaries (SERVER-67385), the clearing of the CSR doesn't actually throw out the contents of the MigrationManager (which really is a reference to the routing table). So in theory, we are leaving more memory behind than we were leaving before upon drop of a sharded collection.

Under this ticket we should make the dropping of a collection to entirely throw out the CSS entry rather than just clear it.