Remove unused language #![feature]s from alloc.

Author: m-ou-se

library/alloc/src/lib.rs

@@ -131,16 +131,12 @@
 // Language features:
 #![feature(allocator_internals)]
 #![feature(allow_internal_unstable)]
-#![feature(arbitrary_self_types)]
 #![feature(associated_type_bounds)]
-#![feature(auto_traits)]
-#![feature(box_patterns)]
 #![feature(box_syntax)]
 #![feature(cfg_sanitize)]
 #![feature(cfg_target_has_atomic)]
 #![feature(const_fn_trait_bound)]
 #![feature(const_trait_impl)]
-#![feature(decl_macro)]
 #![feature(destructuring_assignment)]
 #![feature(dropck_eyepatch)]
 #![feature(exclusive_range_pattern)]
@@ -150,7 +146,6 @@
 #![feature(min_specialization)]
 #![feature(negative_impls)]
 #![feature(never_type)]
-#![feature(nll)]
 #![feature(rustc_allow_const_fn_unstable)]
 #![feature(rustc_attrs)]
 #![feature(staged_api)]