Drop unsafe field comment

Co-authored-by: Ralf Jung <post@ralfj.de>

Author: Mark-Simulacrum

library/core/src/iter/adapters/map_windows.rs

@@ -40,8 +40,6 @@ struct MapWindowsInner<I: Iterator, const N: usize> {
 /// `self.buffer[self.start..self.start + N]` must be initialized,
 /// with all other elements being uninitialized. This also
 /// implies that `self.start <= N`.
-//
-// FIXME make these unsafe fields once that feature is ready
 struct Buffer<T, const N: usize> {
     buffer: [[MaybeUninit<T>; N]; 2],
     start: usize,