Removes the given listener from the list of listeners, that are notified
whenever the value of the
Observable becomes invalid.
If the given listener has not been previously registered (i.e. it was never added) then this method call is a no-op. If it had been previously added then it will be removed. If it had been added more than once, then only the first occurrence will be removed.
|The listener to remove|
|if the listener is null|