Remove obsolete set-cookie tag

Metatag has a "Set cookie" field that generates this tag:

<meta http-equiv="set-cookie" content="foo=bar" />

This meta has been removed from the spec (because it was dangerous) and modern browsers don't support it anymore:
https://github.com/whatwg/html/issues/3076
https://chromestatus.com/feature/6170540112871424

This tag was deprecated in #3582721: Deprecated obsolete set-cookie tag so now needs to be removed.