Value set: remove array-of-array special case

There should not be a need for `get_reference_set` to consider
array-of-array typed expressions to have an unknown offset when others
firmly have offset zero.

Author: tautschnig

src/pointer-analysis/value_set.cpp

@@ -1331,12 +1331,7 @@ void value_sett::get_reference_set_rec(
      expr.id()==ID_string_constant ||
      expr.id()==ID_array)
   {
-    if(
-      expr.type().id() == ID_array &&
-      to_array_type(expr.type()).element_type().id() == ID_array)
-      insert(dest, expr);
-    else
-      insert(dest, expr, mp_integer{0});
+    insert(dest, expr, mp_integer{0});
 
     return;
   }