stat: remove unused high/low prio struct members

stat: remove unused high/low prio struct members

Now when all users have moved to the new clat_prio_stat arrays,
remove io_u_plat_high_prio, io_u_plat_low_prio, clat_high_prio_stat,
and clat_low_prio_stat, as they are no longer used.

Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Link: https://lore.kernel.org/r/20220203192814.18552-18-Niklas.Cassel@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>