UML: Drop private round_down definition

Already defined in kernel.h. The official version assumes that 'n' is
power of two - which it is in our case.
Signed-off-by: Jan Kiszka <jan.kiszka@web.de>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

arch/um/sys-x86_64/signal.c

@@ -165,8 +165,6 @@ struct rt_sigframe
 	struct _fpstate fpstate;
 };
 
-#define round_down(m, n) (((m) / (n)) * (n))
-
 int setup_signal_stack_si(unsigned long stack_top, int sig,
 			  struct k_sigaction *ka, struct pt_regs * regs,
 			  siginfo_t *info, sigset_t *set)