Make bootstrap build on stable

This is generally a good idea, and will help with being able to build bootstrap
without Python over time as it means we can "just" build with cargo +beta build
rather than needing the user to set environment variables. This is a minor step,
but a necessary one on that road.

src/bootstrap/lib.rs

@@ -103,8 +103,6 @@
 //! More documentation can be found in each respective module below, and you can
 //! also check out the `src/bootstrap/README.md` file for more information.
 
-#![feature(drain_filter)]
-
 use std::cell::{Cell, RefCell};
 use std::collections::{HashMap, HashSet};
 use std::env;