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/sys/unix/stack_overflow.rs

@@ -37,7 +37,6 @@ impl Drop for Handler {
           target_os = "bitrig",
           target_os = "openbsd"))]
 mod imp {
-    use core::prelude::*;
     use sys_common::stack;
 
     use super::Handler;