message: Remove unused thread_locals
diff: //! of that page. Update the rest of the documentation to add the new
  //! feature.
  -use std::cell::Cell;
  use std::env;
  use std::fmt;
  use std::str::FromStr;
  @@ -817,11 +816,6 @@ pub fn channel() -> String {
  .unwrap_or_else(|| String::from("dev"))
  }
  -thread_local!(
  - static NIGHTLY_FEATURES_ALLOWED: Cell<bool> = Cell::new(false);
  - static ENABLE_NIGHTLY_FEATURES: Cell<bool> = Cell::new(false);
  -);
  -
  /// This is a little complicated.
  /// This should return false if:
  /// - this is an artifact of the rustc distribution process for "stable" or for "beta"
  