Remove no-longer-needed allow(dead_code) from the tests

`repr(transparent)` now silences the lint.

tests/ui/consts/transmute-const.rs

@@ -3,7 +3,7 @@
 use std::mem;
 
 #[repr(transparent)]
-struct Foo(#[allow(dead_code)] u32);
+struct Foo(u32);
 
 const TRANSMUTED_U32: u32 = unsafe { mem::transmute(Foo(3)) };
 

tests/ui/layout/unsafe-cell-hides-niche.rs

@@ -17,7 +17,7 @@ use std::sync::{Mutex, RwLock};
 struct Wrapper<T>(#[allow(dead_code)] T);
 
 #[repr(transparent)]
-struct Transparent<T>(#[allow(dead_code)] T);
+struct Transparent<T>(T);
 
 struct NoNiche<T>(UnsafeCell<T>);
 

tests/ui/packed/packed-struct-drop-aligned.rs

@@ -24,7 +24,7 @@ impl<'a> Drop for Aligned<'a> {
 }
 
 #[repr(transparent)]
-struct NotCopy(#[allow(dead_code)] u8);
+struct NotCopy(u8);
 
 #[repr(packed)]
 struct Packed<'a>(NotCopy, Aligned<'a>);