wasi/io: remove dead files

Author: RalfJung

library/std/src/os/wasi/io/fd.rs

@@ -1,5 +1,5 @@
 use crate::mem::size_of;
-use crate::os::wasi::io::RawFd;
+use crate::os::unix::io::RawFd;
 
 #[test]
 fn test_raw_fd_layout() {
@@ -9,3 +9,7 @@ fn test_raw_fd_layout() {
     // to be updated.
     assert_eq!(size_of::<RawFd>(), 4);
 }
+
+// Tests for this module
+#[cfg(test)]
+mod tests;