s390: Remove leftover account_tick_vtime() header

The function doesn't seem to exist anymore.

Signed-off-by: Frederic Weisbecker <[e-mail]>
Acked-by: Martin Schwidefsky <[e-mail]>
Cc: Tony Luck <[e-mail]>
Cc: Fenghua Yu <[e-mail]>
Cc: Benjamin Herrenschmidt <[e-mail]>
Cc: Paul Mackerras <[e-mail]>
Cc: Heiko Carstens <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>

arch/s390/include/asm/switch_to.h

···
 	prev = __switch_to(prev,next);					\
 } while (0)
 
-extern void account_tick_vtime(struct task_struct *);
-
 #define finish_arch_switch(prev) do {					     \
 	set_fs(current->thread.mm_segment);				     \
 } while (0)