We should just drop the unsharded collections that were moved; there is no need to drop the whole database (and anyway, we only do that if there were no sharded collections), particularly since once unsharded collections are tracked as sharded collections with one chunk, it will be incorrect to drop the database.

Changing the FCV 4.0 protocol to not drop the database will prevent us from having to change the protocol in a later release (and therefore handle upgrade/downgrade and mixed-version clusters in that release).