manpage: fix definition of prio and prioclass options

manpage: fix definition of prio and prioclass options

Remove the reference to the hipri_percentage option in the definition of
the prio and prioclass options as hipri_percentage controls the use of
RWF_HIPRI flag which triggers I/O completion polling, which is unrelated
with I/O priority (polling and I/O priority can be used together). This
change is done in both fio man page and HOWTO document.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>