Twenty Thirteen: Remove `twentythirteen_search_form_format()` filter in favor of `add_theme_support()` call for search form, comment form, and comment list markup. Props SergeyBiryukov, fixes #24269; see #23850.

git-svn-id: http://core.svn.wordpress.org/trunk@24421 1a063a9b-81f0-0310-95a4-ce76da25c4cd