Remove test that fails to parse flatten attribute

test_suite/tests/compile-fail/enum-representation/flatten-enum.rs

@@ -1,21 +0,0 @@
-// Copyright 2018 Serde Developers
-//
-// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
-// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
-// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
-// option. This file may not be copied, modified, or distributed
-// except according to those terms.
-
-#[macro_use]
-extern crate serde_derive;
-
-#[derive(Serialize)] //~ ERROR: proc-macro derive panicked
-//~^ HELP: unknown serde variant attribute `flatten`
-enum Foo {
-    #[serde(flatten)]
-    Foo {
-        x: u32,
-    }
-}
-
-fn main() {}