media: i2c: Drop HAS_EVENTS and event handlers

v4l2_subdev_init_finalize() already sets the HAS_EVENTS flag if a
control handler is set, and subdev_do_ioctl() uses
v4l2_ctrl_subdev_subscribe_event() and v4l2_event_subdev_unsubscribe()
as defaults if the subdev doesn't have .(un)subscribe.
Let's drop the HAS_EVENTS flag and event handlers.

Signed-off-by: Tommaso Merciai <tomm.merciai@gmail.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>