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/enum-representation/internal-tuple-variant.stderr

@@ -1,8 +1,8 @@
 error: #[serde(tag = "...")] cannot be used with tuple variants
-  --> $DIR/internal-tuple-variant.rs:12:10
-   |
-12 | #[derive(Serialize)]
-   |          ^^^^^^^^^
+ --> $DIR/internal-tuple-variant.rs:4:10
+  |
+4 | #[derive(Serialize)]
+  |          ^^^^^^^^^
 
 error: aborting due to previous error