Discogs plugin: fix event listener params

Also delete related deprecated comment.

beets/plugins.py

@@ -441,7 +441,6 @@ def send(event, **arguments):
     log.debug(u'Sending event: {0}', event)
     results = []
     for handler in event_handlers()[event]:
-        # Don't break legacy plugins if we want to pass more arguments
         result = handler(**arguments)
         if result is not None:
             results.append(result)

beetsplug/discogs.py

@@ -58,7 +58,7 @@ class DiscogsPlugin(BeetsPlugin):
         self.discogs_client = None
         self.register_listener('import_begin', self.setup)
 
-    def setup(self):
+    def setup(self, session):
         """Create the `discogs_client` field. Authenticate if necessary.
         """
         c_key = self.config['apikey'].get(unicode)