powerpc: remove the last remnants of cputime_t

cputime_t was a core kernel type, removed by commits
ed5c8c85..b672592f. As explained in commit b672592f
("sched/cputime: Remove generic asm headers"), the final cleanup
is for the arch to provide cputime_to_nsec[s](). Commit ade7667a


("powerpc: Add cputime_to_nsecs()") did that, but justdidn't remove
the then-unused cputime_to_usecs(), cputime_t type, and associated
remnants.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221006105653.115829-1-npiggin@gmail.com