Remove early return on terminated state

Code as stands breaks shutdown process. It was a bugfix by Arthur to a
bad line which was testing for an impossible state - but afaics we do
actually want to flush in those states. It is possible I am wrong on
that though - if there are any shutdown issues introduced aroudn this
commit I would suggest further investigation around this flushing
behavior.

src/pipe.cpp

@@ -189,10 +189,6 @@ void zmq::pipe_t::rollback ()
 
 void zmq::pipe_t::flush ()
 {
-    //  If terminate() was already called do nothing.
-    if (state == terminated || state == double_terminated)
-        return;
-
     //  The peer does not exist anymore at this point.
     if (state == terminating)
         return;