Remove the Jetty and Thymeleaf version pins from the root pom #261

jetty.version with the jetty-bom import, and thymeleaf.version with its entry, are used by no module any more, and no Janus code imports either library. They override the versions Oriel brings and have to be kept in step by hand (Thymeleaf 3.1.5 in #259).

Remove them. jcef.version stays, because desktop uses jcefmaven directly, but it has to match Oriel's engine (146.0.10).