kobject: clean up the kobject add documentation a bit more

Commit 1fd7c3b4 ("kobject: Improve doc clarity kobject_init_and_add()")
tried to provide more clarity, but the reference to kobject_del() was
incorrect.  Fix that up by removing that line, and hopefully be more explicit
as to exactly what needs to happen here once you register a kobject with the
kobject core.

Acked-by: Tobin C. Harding <tobin@kernel.org>
Fixes: 1fd7c3b4 ("kobject: Improve doc clarity kobject_init_and_add()")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>