Twenty Fourteen: remove `wp_is_mobile()` call from post thumbnail size logic as it's not intended for front-end use. Props kwight, fixes #26221.
Built from https://develop.svn.wordpress.org/trunk@26599


git-svn-id: http://core.svn.wordpress.org/trunk@26489 1a063a9b-81f0-0310-95a4-ce76da25c4cd