Don't show 'Use as featured image' in the image insert dialog if the post type...

Don't show 'Use as featured image' in the image insert dialog if the post type does not support thumbnails. Props SergeyBiryukov. fixes #18669

git-svn-id: https://core.svn.wordpress.org/trunk@19256 1a063a9b-81f0-0310-95a4-ce76da25c4cd