`panic_handler` is now stable

Fixes the following warning:

"warning: the feature `panic_handler` has been stable since
1.30.0 and no longer requires an attribute to enable"

crates/panic-handler/src/lib.rs

@@ -1,6 +1,6 @@
 // Hack of a crate until rust-lang/rust#51647 is fixed
 
-#![feature(no_core, panic_handler)]
+#![feature(no_core)]
 #![no_core]
 
 extern crate core;

examples/intrinsics.rs

@@ -11,7 +11,6 @@
 #![feature(lang_items)]
 #![feature(start)]
 #![feature(allocator_api)]
-#![feature(panic_handler)]
 #![cfg_attr(windows, feature(panic_unwind))]
 #![no_std]