driver core: class: remove dev_kobj from struct class

The dev_kobj field in struct class is now only written to, but never
read from, so it can be removed as it is useless.

Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Link: https://lore.kernel.org/r/20230331093318.82288-5-gregkh@linuxfoundation.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>