um: header debriding - activate_ipi()

... had been dead for 15 years.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/include/shared/irq_user.h

@@ -20,6 +20,5 @@ void sigio_run_timetravel_handlers(void);
 extern void free_irq_by_fd(int fd);
 extern void deactivate_fd(int fd, int irqnum);
 extern int deactivate_all_fds(void);
-extern int activate_ipi(int fd, int pid);
 
 #endif