SystemTools: Remove outdated preprocessor checks for Haiku

Haiku does not define __BEOS__ anymore, so there is no need to guard
these BeOS specific workaround for Haiku.  The workarounds themselves
are not needed for Haiku as it has much better POSIX compatibility than
BeOS did.

Change-Id: I91a1598df764abe0febc9c3828e8bfd47e089cee