Unregisters the callback from the owned stop state, if any.
The destructor does not block waiting for the execution of another callback
registered by an associated stop_callback.
is concurrently executing on another thread,
then the return from the invocation of callback
strongly happens before ([intro.races]
is executing on the current thread,
then the destructor does not block ([defns.block]
) waiting for
the return from the invocation of callback.
Releases ownership of the stop state, if any.