Remove duplicate feature gate tests

As part of the audit for #22820 the following duplicate feature
gate tests were removed:

* `box_patterns`
* `simd_ffi`

These tests for `box_patterns` and `simd_ffi` were added in #23578,
however there were existing tests in #20723 and #21233 respectively.

src/test/compile-fail/gated-box-patterns.rs

@@ -1,21 +0,0 @@
-// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
-// file at the top-level directory of this distribution and at
-// http://rust-lang.org/COPYRIGHT.
-//
-// 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.
-
-// Test that patterns including the box syntax are gated by `box_patterns` feature gate.
-
-fn main() {
-    let x = Box::new(1);
-
-    match x {
-        box 1 => (),
-        //~^ box pattern syntax is experimental
-        _     => ()
-    };
-}

src/test/compile-fail/gated-simd-ffi.rs

@@ -1,27 +0,0 @@
-// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
-// file at the top-level directory of this distribution and at
-// http://rust-lang.org/COPYRIGHT.
-//
-// 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.
-
-// Test that the use of smid types in the ffi is gated by `smid_ffi` feature gate.
-
-#![feature(simd)]
-
-#[repr(C)]
-#[derive(Copy, Clone)]
-#[simd]
-pub struct f32x4(f32, f32, f32, f32);
-
-#[allow(dead_code)]
-extern {
-    fn foo(x: f32x4);
-    //~^ ERROR use of SIMD type `f32x4` in FFI is highly experimental and may result in invalid code
-    //~| HELP add #![feature(simd_ffi)] to the crate attributes to enable
-}
-
-fn main() {}