Remove unused getters in Marketplace Service and Portlet

Remove unused getters for `portletDefinitionRegistry` and `portletCategoryRegistry` JavaBean properties in `MarketplaceService` and for `marketplaceService` in `PortletMarketplaceController`.

Follows up on feedback at https://github.com/Jasig/uPortal/pull/312#discussion_r12190297 to apply the remove as it applies to classes already in `master`.