In notify-types, relicense src/event.rs to (MIT OR Apache-2.0)

This file was copied from the main notify crate and had a header comment
that indicated it was licensed CC0-1.0 or Artistic-2.0 with an
additional clause (see notify-rs#514);

Since @dfaust indicates in
notify-rs#661 (comment) that
this file can be relicensed, we simply remove the old header comment to
indicate that (by default) it falls under the overall (MIT OR
Apache-2.0) license of the notify-types crate.

notify-types/src/event.rs

@@ -1,5 +1,3 @@
-// This file is dual-licensed under the Artistic License 2.0 as per the
-// LICENSE.ARTISTIC file, and the Creative Commons Zero 1.0 license.
 //! The `Event` type and the hierarchical `EventKind` descriptor.
 
 use std::{