Twenty Thirteen: remove unused selector.

git-svn-id: https://core.svn.wordpress.org/trunk@23538 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Thirteen: make footer widget area the primary sidebar, acting as "secondary" content. Move optional sidebar widget area to "tertiary" content. Props obenland, fixes #23644.

git-svn-id: https://core.svn.wordpress.org/trunk@23539 1a063a9b-81f0-0310-95a4-ce76da25c4cd