Remove the ad feature

Needs a proposal but it seems right to deprecate the ad feature:

Points to code to remove:

• https://github.com/xwiki/xwiki-platform/blob/bd419a583691c1156a38e44022781ae876932da0/xwiki-platform-core/xwiki-platform-web/xwiki-platform-web-templates/src/main/resources/templates/ad.vm
• https://github.com/xwiki/xwiki-platform/blob/acbe8f018d7c935bafe091d38d2f1e5e9c1e432c/xwiki-platform-core/xwiki-platform-oldcore/src/main/java/com/xpn/xwiki/api/XWiki.java#L2337