By default, all targets support the "haiku" platform, and we no longer
support building for BeOS, Dan0, Zeta, or other BeOS-compatible targets,
so this is no longer needed.
Also remove all references to the non-Haiku compatible platforms, and
change all BEOS_COMPATIBLE checks to HAIKU_COMPATIBLE. Removal of
all SetSubDirSupportedPlatformsBeOSCompatible invocations
will be in the next commit.