s390/vtime: Remove unused __ARCH_HAS_VTIME_TASK_SWITCH leftover

__ARCH_HAS_VTIME_TASK_SWITCH macro is not used anymore.

Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Acked-by: Nicholas Piggin <npiggin@gmail.com>
Link: https://lore.kernel.org/r/b1055852eab0ffea33ad16c92d6a825c83037c3e.1712760275.git.agordeev@linux.ibm.com