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

Author: m-ou-se

library/alloc/src/lib.rs

@@ -99,7 +99,6 @@
 #![cfg_attr(not(no_global_oom_handling), feature(const_btree_new))]
 #![feature(const_cow_is_borrowed)]
 #![feature(core_intrinsics)]
-#![feature(cow_is_borrowed)]
 #![feature(dispatch_from_dyn)]
 #![feature(exact_size_is_empty)]
 #![feature(extend_one)]
@@ -111,7 +110,6 @@
 #![feature(layout_for_ptr)]
 #![feature(maybe_uninit_extra)]
 #![feature(maybe_uninit_slice)]
-#![feature(maybe_uninit_uninit_array)]
 #![cfg_attr(test, feature(new_uninit))]
 #![feature(nonnull_slice_from_raw_parts)]
 #![feature(option_result_unwrap_unchecked)]
@@ -120,7 +118,6 @@
 #![feature(receiver_trait)]
 #![feature(set_ptr_value)]
 #![feature(slice_group_by)]
-#![feature(slice_partition_dedup)]
 #![feature(slice_ptr_get)]
 #![feature(slice_ptr_len)]
 #![feature(slice_range)]