driver core: class: remove subsystem private pointer from struct class

Now that the last users of the subsystem private pointer in struct class
are gone, the pointer can be removed, as no one is using it.  One step
closer to allowing struct class to be const and moved into read-only
memory.

Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Link: https://lore.kernel.org/r/20230331093318.82288-3-gregkh@linuxfoundation.org


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