remove deprecated struct_exprt constructor

This has no users and has been deprecated since 2018-09-21.

Author: Daniel Kroening

src/util/std_expr.h

@@ -1882,12 +1882,6 @@ inline union_exprt &to_union_expr(exprt &expr)
 class struct_exprt : public multi_ary_exprt
 {
 public:
-  DEPRECATED(
-    SINCE(2018, 9, 21, "use struct_exprt(component_name, value, type) instead"))
-  struct_exprt() : multi_ary_exprt(ID_struct)
-  {
-  }
-
   DEPRECATED(
     SINCE(2019, 1, 12, "use struct_exprt(component_name, value, type) instead"))
   explicit struct_exprt(const typet &_type) : multi_ary_exprt(ID_struct, _type)