remove an unused callback in test

tiramisu/value.py

@@ -79,6 +79,9 @@ class Values(object):
     def __getitem__(self, opt):
         return self.getitem(opt)
 
+    def get_modified_values(self):
+        return self._values
+
     def getitem(self, opt, validate=True, force_permissive=False,
                 force_properties=None):
         if opt in self._cache: