media: Revert "media: v4l2-fwnode: Add a convenience function for registering subdevs with notifiers"

The users of v4l2_async_register_fwnode_subdev() have switched to
parsing their endpoints and setting up async sub-device lists in their
notifiers locally, without using the endpoint parsing callbacks. There
are no more users of v4l2_async_register_fwnode_subdev() so this
convenience function can be removed.

This reverts commit 1634f0ed.
Signed-off-by: Steve Longerbeam <slongerbeam@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>