Remove a crash-causing line in slipnet.py.

Without this patch, `python main.py abc aabbcc milk` will reliably crash.
I believe what happens here is that we initialize all the slipnodes and
everything, and then `slipnet.predecessor` becomes `None`, which means
that if that concept ever arises on its own (vs. arising as the "opposite"
of "successor"), we'll be passing around `None` instead of a proper `Slipnode`
and everything goes sideways.

This line doesn't correspond obviously to anything in the Java code,
so I think it's just bogus --- an experiment in "brain damage" that was
accidentally committed?

copycat/slipnet.py

@@ -27,7 +27,6 @@ class SlipNet(object):
         self.numberOfUpdates = 0
         self.__addInitialNodes()
         self.__addInitialLinks()
-        self.predecessor = None
 
     def __repr__(self):
         return '<slipnet>'