libexpr: Remove non-const overload of listElems

This overload isn't actually necessary anywhere and
doesn't make much sense. The pointers to `Value`s are
themselves const, but the `Value`s are mutable.
A non-const member function implies that the object itself
can be modified but this doesn't make much sense considering
the return type: `Value * const * `, which is a pointer
to a constant array of pointers to mutable values.

(cherry picked from commit 7b46eb9958)

src/libexpr/include/nix/expr/value.hh

@@ -451,11 +451,6 @@ public:
         return internalType == tList1 || internalType == tList2 || internalType == tListN;
     }
 
-    Value * const * listElems()
-    {
-        return internalType == tList1 || internalType == tList2 ? payload.smallList : payload.bigList.elems;
-    }
-
     std::span<Value * const> listItems() const
     {
         assert(isList());