DEC OSF/1 (aka Tru64 UNIX) has been discontinued a few years ago already[1].

This removes the undoubtedly bitrotten support for `OSOsf3 :: OS` from GHC's
code-base.

Support for `ArchAlpha :: Arch` may be removed at some later point, as there
may still be users out there running a more or less recent Linux/alpha
distribution on their more-than-a-decade old Alpha hardware...

 [1]: https://en.wikipedia.org/wiki/Tru64_UNIX