Remove #![feature(alloc_layout_extra)] (#806)

A subset of this feature was stabilized in rust-lang/rust#69362,
and none of the still-unstable methods are in use in `blog_os`

Author: Aaron1011

src/lib.rs

@@ -4,7 +4,6 @@
 #![feature(abi_x86_interrupt)]
 #![feature(alloc_error_handler)]
 #![feature(const_fn)]
-#![feature(alloc_layout_extra)]
 #![feature(const_in_array_repeat_expressions)]
 #![test_runner(crate::test_runner)]
 #![reexport_test_harness_main = "test_main"]