kobject core: remove rwsem from struct subsystem

It isn't used at all by the driver core anymore, and the few usages of
it within the kernel have now all been fixed as most of them were using
it incorrectly.  So remove it.

Now the whole struct subsys can be removed from the system, but that's
for a later patch...

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