Remove `#![feature(panic_handler)]` (#844)

substrate/core/executor/wasm/src/lib.rs

 #![no_std]
-#![feature(panic_handler)]
 #![cfg_attr(feature = "strict", deny(warnings))]
 
 #![feature(alloc)]

substrate/core/sr-io/src/lib.rs

@@ -20,7 +20,6 @@
 
 #![cfg_attr(not(feature = "std"), no_std)]
 #![cfg_attr(not(feature = "std"), feature(lang_items))]
-#![cfg_attr(not(feature = "std"), feature(panic_handler))]
 #![cfg_attr(not(feature = "std"), feature(alloc_error_handler))]
 #![cfg_attr(not(feature = "std"), feature(core_intrinsics))]
 #![cfg_attr(not(feature = "std"), feature(alloc))]

substrate/core/sr-sandbox/src/lib.rs

@@ -39,7 +39,6 @@
 
 #![warn(missing_docs)]
 #![cfg_attr(not(feature = "std"), no_std)]
-#![cfg_attr(not(feature = "std"), feature(panic_handler))]
 #![cfg_attr(not(feature = "std"), feature(core_intrinsics))]
 #![cfg_attr(not(feature = "std"), feature(alloc))]
 

substrate/core/sr-std/src/lib.rs

@@ -20,7 +20,6 @@
 // end::description[]
 
 #![cfg_attr(not(feature = "std"), no_std)]
-#![cfg_attr(not(feature = "std"), feature(panic_handler))]
 #![cfg_attr(not(feature = "std"), feature(core_intrinsics))]
 #![cfg_attr(not(feature = "std"), feature(alloc))]