std.os: remove special case for haiku

This is an accident from a merge conflict introduced in
7edb204edfa41e11776ac009da5a20fb1c907f5f.

The new pipe2 code I believe is supposed to work for all posix-like
systems. If haiku needs special handling here, it should be
re-introduced.
4360f45d7fAndrew Kelley committed on 2/26/2021, 12:42:14 AMparent153c97a
1 file changedLine totals unavailable