Twenty Thirteen: revert addition of `twentythirteen_attachment_link()` in r23523 -- as discussed in the following ticket comments, the tradeoff means simpler code instead of a minor UX enhancement. Closes #23543 and #23719.

git-svn-id: https://core.svn.wordpress.org/trunk@23677 1a063a9b-81f0-0310-95a4-ce76da25c4cd