Credits: remove the useless songName argument

fofix/game/Credits.py

@@ -105,7 +105,7 @@ def render(self, offset):
 class Credits(Layer, KeyListener):
     """Credits scroller."""
 
-    def __init__(self, engine, songName=None):
+    def __init__(self, engine):
         self.engine      = engine
         self.time        = 0.0
         self.offset      = 0.5  # this seems to fix the delay issue, but I'm not sure why