powerpc: Remove obsolete prototype

The include/asm-powerpc/paca.h file has a prototype for a function that
does not exist any more; its name is setup_boot_paca.  This function was
removed in commit 4ba99b97, so its
prototype should have been removed at that time too.
Signed-off-by: Julio M. Merino Vidal <jmerino@ac.upc.edu>
Signed-off-by: Adrian Bunk <bunk@kernel.org>

include/asm-powerpc/paca.h

@@ -121,7 +121,5 @@ struct paca_struct {
 
 extern struct paca_struct paca[];
 
-void setup_boot_paca(void);
-
 #endif /* __KERNEL__ */
 #endif /* _ASM_POWERPC_PACA_H */