Do not hardcode deny(warnings) in the source code

When a new version of the compiler introduces a new warning
it makes your perfectly fine older release require manual
patching in order to build again.

podcasts-data/src/lib.rs

@@ -62,7 +62,6 @@
     missing_copy_implementations
 )]
 #![allow(proc_macro_derive_resolution_fallback)]
-// #![deny(warnings)]
 
 //! FIXME: Docs
 

podcasts-downloader/src/lib.rs

@@ -39,7 +39,6 @@
     elided_lifetime_in_paths,
     missing_copy_implementations
 )]
-// #![deny(warnings)]
 
 #[macro_use]
 extern crate failure_derive;

podcasts-gtk/src/main.rs

@@ -41,7 +41,6 @@
     elided_lifetime_in_paths,
     missing_copy_implementations
 )]
-// #![deny(warnings)]
 
 #[macro_use]
 extern crate failure;