alloc: Drop unused allocator_api feature

This nightly feature is not required for `GlobalAlloc`.

Allows compiling `redox_uefi_alloc` on a stable toolchain.

Signed-off-by: Tim Crawford <tcrawford@system76.com>

Author: crawfxrd

crates/uefi_alloc/src/lib.rs

@@ -1,4 +1,3 @@
-#![feature(allocator_api)]
 #![no_std]
 
 use core::alloc::{GlobalAlloc, Layout};