Fix build and Maven POMs following rebasing of org.json

Rebased org.json is now bundled within gwt-dev and gwt-servlet and
removed as an external dependency (and from gwt-servlet-deps).

gwt-servlet-deps is left to only contain javax.validation so it's no
longer really needed, but kept for backwards compatibility with scripts
and tools that depend on it (WebAppCreator, Google Plugin for Eclipse,
etc.)

Change-Id: Ie372a39d1d4169a435073e5d3615aa499d38935f