Remove some redundant casts

Summary: These went from the same input to output type, so totally useless. Spotted by a new version of clippy.

Reviewed By: JakobDegen

Differential Revision: D47226538

fbshipit-source-id: 083079305f1411bf9905aa7eec5aef824329093e

Author: ndmitchell

starlark/src/collections/alloca.rs

@@ -38,7 +38,7 @@ struct Buffer {
 
 impl Buffer {
     fn alloc(layout: Layout) -> Buffer {
-        let ptr = unsafe { alloc(layout) as *mut u8 };
+        let ptr = unsafe { alloc(layout) };
         let ptr = NonNull::new(ptr).unwrap();
         Buffer { ptr, layout }
     }

starlark/src/values/layout/heap/arena.rs

@@ -184,7 +184,7 @@ impl<A: ArenaAllocator> Arena<A> {
         unsafe {
             let repr = &mut *(p as *mut MaybeUninit<AValueRepr<T>>);
             let extra = slice::from_raw_parts_mut(
-                (p as *mut u8).add(AValueRepr::<T>::offset_of_extra()) as *mut _,
+                p.add(AValueRepr::<T>::offset_of_extra()) as *mut _,
                 extra_len,
             );
             (repr, extra)