[python-tulip] Is _subprocess_closed method requred?

[Andrew Svetlov]

BaseSubprocessTransport calls event_loop._subprocess_closed() method on 
process exit.
Both unix and windows implementations are empty.
_subprocess_closed is a private method from my perspective and should not 
be called/reused by third party code.
Do we need this at all?
I recall reason why I've introduced this method but now implementation of 
subprocess support was changed.