Remove unused frontend-plugins and frontend-theme dependencies from release pom

The release pom still defines two properties (hippo.frontend.plugins.version and hippo.frontend.theme.version) that are no longer used. The 'plugins' were already not used anymore (ancient dependency, nobody bothered to clean it up). The 'theme' has now been un-mavenized since it moved to NPM (HIPPOTHEME-166). So both properties and the dependencies in the dependencyManagement section that used these properties can be removed from the release pom.