Comment out arithmetic_simplify

arithmetic_simplify makes test_pwnable_collision in manticore-examples
repo hang. I'm not exactly sure what is happening, but there's probably
infinite recursion/walking/"simplification" happening somewhere.

manticore/core/smtlib/visitors.py

@@ -788,7 +788,8 @@ def to_constant(expression):
 
 @lru_cache(maxsize=128, typed=True)
 def simplify(expression):
-    expression = arithmetic_simplify(expression)
+    # TODO: Fix arithmetic_simplify
+    # expression = arithmetic_simplify(expression)
     return expression