std: Remove #[allow] directives in sys modules

These were suppressing lots of interesting warnings! Turns out there was also
quite a bit of dead code.

src/libstd/lib.rs

@@ -273,13 +273,14 @@ pub mod collections;
 pub mod thread;
 pub mod sync;
 
+#[macro_use]
+#[path = "sys/common/mod.rs"] mod sys_common;
+
 #[cfg(unix)]
 #[path = "sys/unix/mod.rs"] mod sys;
 #[cfg(windows)]
 #[path = "sys/windows/mod.rs"] mod sys;
 
-#[path = "sys/common/mod.rs"] mod sys_common;
-
 pub mod rt;
 mod panicking;