Remove a case that is no longer reachable in encodings.py.

This case was originally designed to handle Exception's which didn't gracefully support coercing themselves to unicode. However, because it lives in the `else` case of `if hasattr(s, '__unicode__'):` we can be sure it's no longer reachable in djanog anymore, because since Python 2.5 exception has subclassed object, which means Exception objects always have an __unicode__ method.