media: atomisp: Remove atomisp_[sub]dev_users()

The atomisp_[sub]dev_users() functions are not used anymore,
remove them.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>