Twenty Fourteen: remove the pattern image used as a placeholder for featured images. This greatly improves the "first run" experience of the theme and gives it a more general appeal as a blog theme by putting less of a requirement on featured images. Props iamtakashi, fixes #25859.

git-svn-id: https://develop.svn.wordpress.org/trunk@26034 602fd350-edb4-49c9-b593-d223f7449a82