driver core: remove the driver-model structures from the documentation

Remove the struct bus_type, class, device, device_driver from the
driver-model docs. With another patch add them to device.h, since
they are out of date. That will keep things up to date and provide
a better way to document this stuff.

Signed-off-by: Wanlong Gao <wanlong.gao@gmail.com>
Acked-by: Harry Wei <harryxiyou@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>