media: i2c: hi556: Drop system suspend and resume handlers

Stopping streaming on a camera pipeline at system suspend time, and
restarting it at system resume time, requires coordinated action between
the bridge driver and the camera sensor driver. This is handled by the
bridge driver calling the sensor's .s_stream() handler at system suspend
and resume time. There is thus no need for the sensor to independently
implement system sleep PM operations. Drop them.

The streaming field of the driver's private structure is now unused,
drop it as well.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>