remove unnecessary feature attributes

experiments/src/main.rs

@@ -1,7 +1,6 @@
 #![no_std]
 #![no_main]
 #![feature(naked_functions)]
-#![feature(asm)]
 
 extern crate alloc;
 

libcortex_a9/src/lib.rs

@@ -1,7 +1,5 @@
 #![no_std]
 #![feature(never_type)]
-#![feature(asm, global_asm)]
-#![feature(const_fn_trait_bound)]
 
 extern crate alloc;
 

libsupport_zynq/src/lib.rs

@@ -1,9 +1,7 @@
 #![no_std]
 
 #![feature(alloc_error_handler)]
-#![feature(panic_info_message)]
 #![feature(naked_functions)]
-#![feature(asm)]
 
 pub extern crate alloc;
 pub extern crate compiler_builtins;