api: fix evaluation of elementary functions

Author: mloubout

devito/finite_differences/differentiable.py

@@ -548,9 +548,6 @@ class DifferentiableFunction(DifferentiableOp):
     def __new__(cls, *args, **kwargs):
         return cls.__sympy_class__.__new__(cls, *args, **kwargs)
 
-    def _eval_at(self, func):
-        return self
-
 
 class Add(DifferentiableOp, sympy.Add):
     __sympy_class__ = sympy.Add