Remove nullary_exprt::move_to_operands deletion

There are no other instances of `move_to_operands` across the code base
and no implementation here. Therefore this removal is clean-up assuming
that this member function has been removed from everywhere else and this
instance has accidentally been left behind.

Author: thomasspriggs

src/util/std_expr.h

@@ -39,10 +39,6 @@ class nullary_exprt : public expr_protectedt
   const exprt &op3() const = delete;
   exprt &op3() = delete;
 
-  void move_to_operands(exprt &) = delete;
-  void move_to_operands(exprt &, exprt &) = delete;
-  void move_to_operands(exprt &, exprt &, exprt &) = delete;
-
   void copy_to_operands(const exprt &expr) = delete;
   void copy_to_operands(const exprt &, const exprt &) = delete;
   void copy_to_operands(const exprt &, const exprt &, const exprt &) = delete;