Remove unneeded code

These functions are already provided by unary_exprt, which
typecase_exprt and not_exprt derive from.

Author: Owen Jones

src/util/std_expr.h

@@ -1968,16 +1968,6 @@ class typecast_exprt:public unary_exprt
     unary_exprt(ID_typecast, op, _type)
   {
   }
-
-  exprt &op()
-  {
-    return op0();
-  }
-
-  const exprt &op() const
-  {
-    return op0();
-  }
 };
 
 /*! \brief Cast a generic exprt to a \ref typecast_exprt
@@ -3035,16 +3025,6 @@ class not_exprt:public unary_exprt
   not_exprt():unary_exprt(ID_not, bool_typet())
   {
   }
-
-  exprt &op()
-  {
-    return op0();
-  }
-
-  const exprt &op() const
-  {
-    return op0();
-  }
 };
 
 /*! \brief Cast a generic exprt to an \ref not_exprt