Move xwiki-platform-oldrendering to contrib

Following the work done on XWIKI-12001 to make the old rendering engine (xwiki/1.0 syntax based renderer) installable as extension I would like to propose to move it to https://github.com/xwiki-contrib/ in 8.0 branch.

See http://markmail.org/message/fg6d7zo4wb7nqmc7.