Nuke COMPILE_FOR_R5.

Not used in $long_time, almost certainly broken, so it's just cruft.

headers/build/os/drivers/Select.h

@@ -1,5 +1,5 @@
 /* File System and Drivers select support
-** 
+**
 ** Distributed under the terms of the Haiku License.
 */
 #ifndef _DRIVERS_SELECT_H
@@ -31,11 +31,7 @@ enum select_events {
 extern "C" {
 #endif
 
-#ifdef COMPILE_FOR_R5
-extern void notify_select_event(struct selectsync *sync, uint32 ref);
-#else
 extern status_t notify_select_event(struct selectsync *sync, uint32 ref, uint8 event);
-#endif
 
 #ifdef __cplusplus
 }