Following recent changes to use libroot_build on Haiku also, it is now
actually impossible to build Haiku components on non-Haiku platforms
(BeOS R5, Dan0, BONE, Zeta), so we can remove any logic related to this.
This is only the first part; still to be removed are:
* SetSubDirSupportedPlatformsBeOSCompatible
* HOST_PLATFORM_BEOS_COMPATIBLE
* TARGET_PLATFORM_BEOS_COMPATIBLE