Revert "USB: usb-skeleton.c: fix open/disconnect race"

This reverts commit 26c71a79.

It's not needed, to quote Ming Lei:
	Looks you have queued the patch into your tree, but just now I
	find the patch is not needed at all, since we have had
	minor_rwsem(drivers/usb/core/file.c) for this purpose, please
	drop the patch, sorry for it.

Cc: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>