Restore the original z-index of the sticky admin menu, as first set in [26442].

The inline comment is now correct again. This z-index was changed in [26701], but those circumstances no longer apply after [27532].

props mordauk.
see #26442, #26952.
fixes #26567.

Built from https://develop.svn.wordpress.org/trunk@27833


git-svn-id: http://core.svn.wordpress.org/trunk@27667 1a063a9b-81f0-0310-95a4-ce76da25c4cd