Deletes the given import by its importId.

Performs deletion of all events in the import given by its importId. Please note that
this is asynchronous operation that is performed on the background and depending on the
number of events imported so far it may take a while to finish.

It is recommended on the client/developer side that the verifies that the importId was deleted by querying the list
of the available import ids before sending new events after delete. If events were sent during ongoing
delete background operation then their state with regards to their persistence is undefined.

Using entirely new importId after delete is recommended.

Returns HTTP 204 even if the importId does not exist or was already deleted.