driver core: remove owner field from struct bus_type

This isn't used by anything in the driver core, and by no one in the 204
different usages of it in the kernel tree.  Remove this field so no one
gets any idea that it is needed to be used.

Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>