Ensuring Runnables posted with delay to a Handler are removed when unsubscribed.

This patch ensures the delayed runnables posted to a Handler are properly
removed when Subscription.unsubscribe() is called on the Observable.

The original code returns the subscription from schedule() but is not used
by the callers who instead add the Worker itself as a subsciption.

Signed-off-by: David Marques <dpsmarques@gmail.com>