Simplify volume on base playback

mopidy/backends/__init__.py

@@ -96,11 +96,11 @@ class BasePlaybackController(object):
     repeat = False
     random = False
     consume = False
+    volume = None
     
     def __init__(self, backend):
         self.backend = backend
         self.current_track = None
-        self._volume = None
 
     def play(self, id=None, position=None):
         raise NotImplementedError
@@ -186,10 +186,6 @@ class BasePlaybackController(object):
         except ValueError:
             return None
 
-    @property
-    def volume(self):
-        return self._volume
-
     @property
     def time_position(self):
         raise NotImplementedError