Revert "Fix Server to wait in wait_closed() until all transports are …

…done" (#180)

This reverts commit 124e981.

uvloop/server.pyx

@@ -44,7 +44,7 @@ cdef class Server:
 
     @cython.iterable_coroutine
     async def wait_closed(self):
-        if self._waiters is None:
+        if self._servers is None or self._waiters is None:
             return
         waiter = self._loop._new_future()
         self._waiters.append(waiter)