Revert stabilizing integer::BITS.

library/alloc/src/lib.rs

@@ -102,6 +102,7 @@
 #![feature(fn_traits)]
 #![feature(fundamental)]
 #![feature(inplace_iteration)]
+#![feature(int_bits_const)]
 #![feature(lang_items)]
 #![feature(layout_for_ptr)]
 #![feature(maybe_uninit_ref)]