Remove unused IDESCSI_PC_RQ while at it.

There should be no functionality change resulting from this patch.

[bart: IDE_FLAG_* -> IDE_AFLAG_*, dev_flags -> atapi_flags]
Signed-off-by: Borislav Petkov <petkovbb@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>