Remove useless run-pass test

When originally added, this test used to contain a `#![plugin(clippy)]`.
This was removed at some point along the way, at which point this test
no longer tests anything. It prints:

    warning: unknown lint: `identity_op`
     --> src/main.rs:1:9
      |
    1 | #![deny(identity_op)]
      |         ^^^^^^^^^^^
      |
      = note: #[warn(unknown_lints)] on by default

which is swallowed and ignored by compiletest.

Nowadays Clippy handles warnings inside of macro expanded code
intelligently and this is something they would be responsible for
testing.

test_suite/tests/compiletest.rs

@@ -29,8 +29,3 @@ fn run_mode(mode: &'static str) {
 fn compile_fail() {
     run_mode("compile-fail");
 }
-
-#[test]
-fn run_pass() {
-    run_mode("run-pass");
-}

test_suite/tests/run-pass/identity-op.rs

@@ -1,22 +0,0 @@
-// Copyright 2017 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.
-
-#![deny(identity_op)]
-
-#[macro_use]
-extern crate serde_derive;
-
-// The derived implementation uses 0+1 to add up the number of fields
-// serialized, which Clippy warns about. If the expansion info is registered
-// correctly, the Clippy lint is not triggered.
-#[derive(Serialize)]
-struct A {
-    b: u8,
-}
-
-fn main() {}