[media] atomisp2: remove HRT_UNSCHED

HRT_UNSCHED is never defined or set in the driver, so this is dead code that
can be retired, simplifying the code a bit further.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>