Removed SkinTemplateSetupPageCss hook and SkinTemplate::setupPageCss(), no extension...

Removed SkinTemplateSetupPageCss hook and SkinTemplate::setupPageCss(), no extension use this anymore. This can only add raw CSS to page; it is much better to use the ResourceLoader (or to link to a file). But if it is really needed, extensions can use the BeforePageDisplay to achieve the same functionnality.