Remove erroneous license header from lib.rs.

This crate has always been licensed under Apache2/MIT as specified in Cargo.toml.
The license header text in this file was an oversight on my part.

Author: bholley

src/lib.rs

@@ -1,7 +1,3 @@
-/* This Source Code Form is subject to the terms of the Mozilla Public
- * License, v. 2.0. If a copy of the MPL was not distributed with this
- * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
-
 //! Implements a container type providing RefCell-like semantics for objects
 //! shared across threads.
 //!