tidyup around artemis-website module used to generate docs for website

The artemis-website module is used to build various documentation (user manual, developer guide, migration guide, javadocs) used to update the content of the standalone activemq-website repo, particularly during releases. Historically some of the artemis-website output was actually used by the binary distribution, but that is no longer the case. Various cruft remains in the build due to this, it can now be removed to simplify the module and also make the remaining build and output clearer.