sched/vtime: Remove confusing arch_vtime_task_switch() declaration

Callback arch_vtime_task_switch() is only defined when
CONFIG_VIRT_CPU_ACCOUNTING_NATIVE is selected. Yet, the
function prototype forward declaration is present for
CONFIG_VIRT_CPU_ACCOUNTING_GEN variant. Remove it.

Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Link: https://lore.kernel.org/r/783d7c611864f82b0fb9edf01890b9396f3a549a.1712760275.git.agordeev@linux.ibm.com

Author: Alexander Gordeev

include/linux/vtime.h

@@ -18,7 +18,6 @@ extern void vtime_account_idle(struct task_struct *tsk);
 #endif /* !CONFIG_VIRT_CPU_ACCOUNTING */
 
 #ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
-extern void arch_vtime_task_switch(struct task_struct *tsk);
 extern void vtime_user_enter(struct task_struct *tsk);
 extern void vtime_user_exit(struct task_struct *tsk);
 extern void vtime_guest_enter(struct task_struct *tsk);