Use correct class. see #23933.

git-svn-id: http://core.svn.wordpress.org/trunk@23915 1a063a9b-81f0-0310-95a4-ce76da25c4cd
In the HTML outputted from get_search_form(): remove incrementing the IDs for XHTML after the first call, remove the <div> wrapper for HTML5. Props obenland, props bpetty. Fixes #16539

git-svn-id: http://core.svn.wordpress.org/trunk@23916 1a063a9b-81f0-0310-95a4-ce76da25c4cd