s390: remove the last remnants of cputime_t

s390: remove the last remnants of cputime_t

cputime_t was a core kernel type, removed by commits
ed5c8c854f2b..b672592f0221. As explained in commit b672592f0221
("sched/cputime: Remove generic asm headers"), the final cleanup is for
the arch to provide cputime_to_nsec[s](). Commit e53051e757d6
("s390/cputime: provide archicture specific cputime_to_nsecs") did that,
but just didn't remove the then-unused cputime_to_usecs() and associated
remnants.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Link: https://lore.kernel.org/r/20221006105635.115775-1-npiggin@gmail.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>