Revert "kobject: introduce kobj_completion"

This reverts commit eee03164.

Jeff writes:
	I have no objections to reverting it. There were concerns from
	Al Viro that it'd be tough to get right by callers and I had
	assumed it got dropped after that. I had planned on using it in
	my btrfs sysfs exports patchset but came up with a better way.

Cc: Jeff Mahoney <jeffm@suse.com>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>