message: remove some no-longer-needed allow(const_err)
diff: -// allow(const_err) to work around a bug in warnings
  -#[allow(const_err)]
  static FOO: fn() = || { assert_ne!(42, 43) };
  -#[allow(const_err)]
  static BAR: fn(i32, i32) = |a, b| { assert_ne!(a, b) };
  // use to first make the closure FnOnce() before making it fn()
  