Copyright/license headers

The following changes are included:

- Delete per-file license notices at the top of each file.
- Delete the first paragraph of LICENSE-MIT (an inaccurate
  pseudo-copyright line), leaving only the text of the MIT license.

Nothing about the license of Serde code has changed, only our
understanding of how to correctly communicate that license has changed.

This mirrors an equivalent change being applied in the rust-lang/rust
repository.

test_suite/tests/ui/with-variant/skip_de_whole_variant.rs

@@ -1,11 +1,3 @@
-// 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.
-
 #[macro_use]
 extern crate serde_derive;