This commit fixes a panic that could occur when registering N subscriptions for
N entities of the same kind. Before, when dropping the first of the
subscriptions, we would remove the entity ID extractor as well. This was,
however, used by all the other N - 1 subscriptions which would then start
losing messages. In addition, dropping yet another subscription of that kind
would result in a panic, because we wouldn't find the extractor in the map
upon invoking `Subscription::drop`.

With this change we will avoid removing the ID extractor when dropping a
subscription. Crucially, we also avoid inserting extractors for simple message
subscriptions. This enables these non-entity subscriptions to be dropped and
re-registered without seeing a "registered handler for the same message twice"
panic.