proc/stat: remove arch_idle_time()

The last (only) architecture specific arch_idle_time() implementation was
removed with commit be76ea61 ("s390/idle: remove arch_cpu_idle_time()
and corresponding code").

Therefore remove the now dead code in fs/proc/stat.c as well.

Link: https://lkml.kernel.org/r/20230405143452.2677172-1-hca@linux.ibm.com


Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>