Removal of non-Haiku target platform logic from build system (part 1.)

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

headers/private/shared/cpu_type.h

@@ -513,15 +513,8 @@ get_cpu_type(char *vendorBuffer, size_t vendorSize, char *modelBuffer,
 	if (model == NULL)
 		model = "Unknown";
 
-#ifdef R5_COMPATIBLE
-	strncpy(vendorBuffer, vendor, vendorSize - 1);
-	vendorBuffer[vendorSize - 1] = '\0';
-	strncpy(modelBuffer, model, modelSize - 1);
-	modelBuffer[modelSize - 1] = '\0';
-#else
 	strlcpy(vendorBuffer, vendor, vendorSize);
 	strlcpy(modelBuffer, model, modelSize);
-#endif
 }