Remove a redundant "opposite" link from the slipnet.

This does change the micro behavior of Copycat. I would hope it doesn't
change the macro behavior, or at least changes it for the better.

copycat/slipnet.py

@@ -152,7 +152,6 @@ class Slipnet(object):
         opposites = [
             (self.first, self.last),
             (self.leftmost, self.rightmost),
-            (self.leftmost, self.rightmost),
             (self.left, self.right),
             (self.successor, self.predecessor),
             (self.successorGroup, self.predecessorGroup),