Remove "the hackiest hack that ever did hack" from add_user()....

Remove "the hackiest hack that ever did hack" from add_user(). wp_insert_user()/edit_user() can deal with adding new users of any defined role. Fixes #18749.


git-svn-id: https://core.svn.wordpress.org/trunk@19686 1a063a9b-81f0-0310-95a4-ce76da25c4cd