Fix stable_features warning

```
error: the feature `cfg_target_has_atomic` has been stable since 1.60.0 and no longer requires an attribute to enable
 --> futures/tests/no-std/src/lib.rs:3:12
  |
3 | #![feature(cfg_target_has_atomic)]
  |            ^^^^^^^^^^^^^^^^^^^^^
```

Author: taiki-e

futures/tests/no-std/src/lib.rs

@@ -1,6 +1,5 @@
 #![cfg(nightly)]
 #![no_std]
-#![feature(cfg_target_has_atomic)]
 
 #[cfg(feature = "futures-core-alloc")]
 #[cfg(target_has_atomic = "ptr")]