client: fix Subscription.done crash and drop unbacked .empty property

src/p4p/client/asyncio.py

@@ -340,12 +340,8 @@ def close(self):
     @property
     def done(self):
         'Has all data for this subscription been received?'
-        return self._S is None or self._S.done()
+        return self._S is None or self._S.complete()
 
-    @property
-    def empty(self):
-        'Is data pending in event queue?'
-        return self._S is None or self._S.empty()
 
     async def wait_closed(self):
         """Wait until subscription is closed.

src/p4p/client/cothread.py

@@ -260,12 +260,8 @@ def close(self):
     @property
     def done(self):
         'Has all data for this subscription been received?'
-        return self._S is None or self._S.done()
+        return self._S is None or self._S.complete()
 
-    @property
-    def empty(self):
-        'Is data pending in event queue?'
-        return self._S is None or self._S.empty()
 
     def _event(self):
         if self._S is not None:

src/p4p/client/thread.py

@@ -77,12 +77,8 @@ def __exit__(self, A, B, C):
     @property
     def done(self):
         'Has all data for this subscription been received?'
-        return self._S is None or self._S.done()
+        return self._S is None or self._S.complete()
 
-    @property
-    def empty(self):
-        'Is data pending in event queue?'
-        return self._S is None or self._S.empty()
 
     def _event(self):
         try: