media: intel/ipu6: remove buttress ish structure

The buttress ipc ish structure is not effectively used on IPU6 - data
is nullified on init. Remove the ish structure and handing of related
interrupts to cleanup the code.

Signed-off-by: Stanislaw Gruszka <stanislaw.gruszka@linux.intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>