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

`repr(transparent)` now silences the lint.

src/tools/miri/tests/fail/issue-miri-1112.rs

@@ -1,7 +1,7 @@
 trait Empty {}
 
 #[repr(transparent)]
-pub struct FunnyPointer(#[allow(dead_code)] dyn Empty);
+pub struct FunnyPointer(dyn Empty);
 
 #[repr(C)]
 pub struct Meta {

src/tools/miri/tests/fail/unaligned_pointers/drop_in_place.rs

@@ -1,7 +1,7 @@
 //@compile-flags: -Cdebug-assertions=no
 
 #[repr(transparent)]
-struct HasDrop(#[allow(dead_code)] u8);
+struct HasDrop(u8);
 
 impl Drop for HasDrop {
     fn drop(&mut self) {}