Remove a useless allowed attr

Author: tesuji

library/core/src/slice/mod.rs

@@ -66,7 +66,6 @@ impl<T> [T] {
     #[rustc_const_stable(feature = "const_slice_len", since = "1.32.0")]
     #[inline]
     // SAFETY: const sound because we transmute out the length field as a usize (which it must be)
-    #[allow(unused_attributes)]
     #[allow_internal_unstable(const_fn_union)]
     pub const fn len(&self) -> usize {
         // SAFETY: this is safe because `&[T]` and `FatPtr<T>` have the same layout.

library/std/src/os/raw/mod.rs

@@ -9,7 +9,6 @@
 #![stable(feature = "raw_os", since = "1.1.0")]
 
 #[cfg(test)]
-#[allow(unused_imports)]
 mod tests;
 
 #[doc(include = "char.md")]

library/std/src/os/raw/tests.rs

@@ -1,5 +1,4 @@
 use crate::any::TypeId;
-use crate::mem;
 
 macro_rules! ok {
     ($($t:ident)*) => {$(