LibJS: Remove the NativeProperty mechanism from LibJS

These were an ad-hoc way to implement special behaviour when reading or
writing to specific object properties. Because these were effectively
replaced by the abillity to override the internal methods of Object,
they are no longer needed.

Userland/Libraries/LibJS/Runtime/Value.cpp

@@ -318,8 +318,6 @@ String Value::to_string_without_side_effects() const
         return String::formatted("[object {}]", as_object().class_name());
     case Type::Accessor:
         return "<accessor>";
-    case Type::NativeProperty:
-        return "<native-property>";
     default:
         VERIFY_NOT_REACHED();
     }