Remove trivial to remove uses of atomic-polyfill.

embassy-stm32/src/flash/asynch.rs

@@ -1,6 +1,6 @@
 use core::marker::PhantomData;
+use core::sync::atomic::{fence, Ordering};
 
-use atomic_polyfill::{fence, Ordering};
 use embassy_hal_common::drop::OnDrop;
 use embassy_hal_common::into_ref;
 use embassy_sync::blocking_mutex::raw::CriticalSectionRawMutex;