Remove alleged support for BeOS

Since the initial addition of BeOS support in 1999, there has only
been one update to it (in 2005, and it wasn't even very big). GLib is
known to not currently build on Haiku (or presumably actual BeOS)
without additional patching, and the fact that there isn't a single
G_OS_BEOS check in gio/ is suspicious.

Additionally, other than the GModule implementation, all of the
existing G_OS_BEOS checks are either (a) "G_OS_UNIX || G_OS_BEOS", or
(b) random minor POSIXy tweaks (include this header file rather than
that one, etc), suggesting that if we were going to support Haiku, it
would probably be simpler to treat it as a special kind of G_OS_UNIX
(as we do with Mac OS X) rather than as its own completely different
thing.

So, kill G_OS_BEOS.

https://bugzilla.gnome.org/show_bug.cgi?id=710519