use_extern_macros stabilization

With recent nightlies rustc produces a warning

```
the feature `use_extern_macros` has been stable since 1.30.0 and no longer requires an attribute to enable
```

Author: pepyakin

substrate/runtime-std/src/lib.rs

@@ -21,7 +21,6 @@
 #![cfg_attr(not(feature = "std"), feature(panic_implementation))]
 #![cfg_attr(not(feature = "std"), feature(core_intrinsics))]
 #![cfg_attr(not(feature = "std"), feature(alloc))]
-#![cfg_attr(not(feature = "std"), feature(use_extern_macros))]
 
 #![cfg_attr(feature = "std", doc = "Polkadot runtime standard library as compiled when linked with Rust's standard library.")]
 #![cfg_attr(not(feature = "std"), doc = "Polkadot's runtime standard library as compiled without Rust's standard library.")]