remove unused property 'symmetric'

nifty/operators/laplace_operator.py

@@ -80,10 +80,6 @@ class LaplaceOperator(EndomorphicOperator):
     def unitary(self):
         return False
 
-    @property
-    def symmetric(self):
-        return False
-
     @property
     def self_adjoint(self):
         return False

nifty/operators/smoothness_operator.py

@@ -44,10 +44,6 @@ class SmoothnessOperator(EndomorphicOperator):
     def unitary(self):
         return False
 
-    @property
-    def symmetric(self):
-        return False
-
     @property
     def self_adjoint(self):
         return False