Remove support for <link rel=start>, end, up, and index. These rel="" values...

Remove support for <link rel=start>, end, up, and index. These rel="" values have been dropped by the HTML Working Group. props Elpie, kawauso, fixes #18128.

git-svn-id: https://core.svn.wordpress.org/trunk@18680 1a063a9b-81f0-0310-95a4-ce76da25c4cd