Revert stabilizing integer::BITS.

compiler/rustc_serialize/src/lib.rs

@@ -17,6 +17,7 @@ Core encoding and decoding interfaces.
 #![feature(min_specialization)]
 #![feature(vec_spare_capacity)]
 #![feature(core_intrinsics)]
+#![feature(int_bits_const)]
 #![feature(maybe_uninit_slice)]
 #![feature(new_uninit)]
 #![cfg_attr(test, feature(test))]