Callback delegate, invoked when a file descriptor should be removed from the set of file descriptors being monitored for events.
            
- fd (Int32)
 - The file descriptor to stop monitoring.
 
- user_data (IntPtr)
 - User data pointer specified in SetPollfdNotifiers(MonoUsbSessionHandle, PollfdAddedDelegate, PollfdRemovedDelegate, IntPtr) call.
 
After returning from this callback, do not use that file descriptor again.