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