Remove the Maven plugin from the distribution

As discussed on the dev list, move the Maven plugin to the sandbox because no one is maintaining it.

http://www.nabble.com/Moving-the-maven-continuum-plugin-module-to-the-sandbox--td24157206.html

http://www.nabble.com/Trouble-building-the-Continuum-Plugin-docs--td19244617.html#a19245010