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

components/style/lib.rs

@@ -23,7 +23,6 @@
 //! [cssparser]: ../cssparser/index.html
 //! [selectors]: ../selectors/index.html
 
-#![deny(warnings)]
 #![deny(missing_docs)]
 
 #![recursion_limit = "500"]  // For define_css_keyword_enum! in -moz-appearance