<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def remove_done_callback(self, func):
 """Remove a callback from the long running operation. :param callable func: The function to be removed from the callbacks. :raises: ValueError if the long running operation has already completed. """

        if self._done is None or self._done.is_set():
            raise ValueError("Process is complete.")
        self._callbacks = [c for c in self._callbacks if c != func]