Only show top-level items when adding pages to a User's First Nav Menu. (This is also the title of Dave's first children's book.)

props lessbloat.
fixes #25122.

Built from https://develop.svn.wordpress.org/trunk@25622


git-svn-id: http://core.svn.wordpress.org/trunk@25539 1a063a9b-81f0-0310-95a4-ce76da25c4cd