Remove unused #![feature]s from std.

Author: m-ou-se

library/std/src/lib.rs

@@ -229,9 +229,7 @@
 #![feature(allow_internal_unstable)]
 #![feature(array_error_internals)]
 #![feature(assert_matches)]
-#![feature(associated_type_bounds)]
 #![feature(async_iterator)]
-#![feature(atomic_mut_ptr)]
 #![feature(bench_black_box)]
 #![feature(box_syntax)]
 #![feature(c_unwind)]
@@ -251,7 +249,6 @@
 #![feature(const_ipv4)]
 #![feature(const_ipv6)]
 #![feature(const_mut_refs)]
-#![feature(const_option)]
 #![feature(const_socketaddr)]
 #![feature(const_trait_impl)]
 #![feature(c_size_t)]
@@ -307,7 +304,6 @@
 #![feature(raw_os_nonzero)]
 #![feature(rustc_attrs)]
 #![feature(saturating_int_impl)]
-#![feature(slice_internals)]
 #![feature(slice_ptr_get)]
 #![feature(staged_api)]
 #![feature(std_internals)]