Unregisters a shell that was previously registered. After this method completes, the shell will be treated as if it had never been registered at all. If you have registered a shell, you should ensure that this method is called when the shell is disposed. Otherwise, a potential memory leak will exist.
If the shell was never registered, or if the shell is null, then this method returns false and does nothing.
@param shell The shell to be unregistered; does nothing if this value is null.
@return true if the shell had been registered;false otherwise.