Remove remaining cfg(target_has_atomic) (#2487)

Author: taiki-e

futures/src/lib.rs

@@ -141,7 +141,6 @@ pub use futures_util::{future, sink, stream, task};
 #[cfg(feature = "async-await")]
 pub use futures_util::stream_select;
 
-#[cfg_attr(feature = "cfg-target-has-atomic", cfg(target_has_atomic = "ptr"))]
 #[cfg(feature = "alloc")]
 #[doc(inline)]
 pub use futures_channel as channel;