Clean up inconsistent @Override annotations

Our modules are using @Override annotations a bit inconsistently. We recently changed the JDK level for 'modeshape-jcr' and below (see MODE-1108), but we didn't remove the @Override annotations that were implementing interfaces (which is a JDK6 thing). Also, we have quite a few modules that are using JDK6 compiler settings but do not have @Override annotations.