Remove #![deny(warnings)]

We already have https://github.com/servo/servo/pull/19612
to deny warnings at the time of landing into master.
But it’s not useful to break the build when later compiler
with a more recent Rust version that has introduced new warnings:

https://bugzilla.mozilla.org/show_bug.cgi?id=1434619

support/gecko/nsstring/src/lib.rs

@@ -113,7 +113,6 @@
 //! which invoke their member's destructors through C++ code.
 
 #![allow(non_camel_case_types)]
-#![deny(warnings)]
 
 #[macro_use]
 extern crate bitflags;