v0.1.x: Don't deny warnings (#1368)

This is just too aggressive for a stable maintenance branch of tokio,
in that new rust release warnings are prooving too hard to fix.

tokio-sync/src/lib.rs

@@ -1,6 +1,5 @@
 #![doc(html_root_url = "https://docs.rs/tokio-sync/0.1.6")]
 #![deny(missing_debug_implementations, missing_docs, unreachable_pub)]
-#![cfg_attr(test, deny(warnings))]
 
 //! Asynchronous synchronization primitives.
 //!