Issue #2127725 by ParisLiakos: Remove category handling from aggregator.

core/modules/aggregator/aggregator.install

core/modules/aggregator/aggregator.local_actions.yml

core/modules/aggregator/aggregator.local_tasks.yml

core/modules/aggregator/aggregator.module

core/modules/aggregator/aggregator.pages.inc

core/modules/aggregator/aggregator.routing.yml

core/modules/aggregator/aggregator.services.yml

core/modules/aggregator/aggregator.views.inc

core/modules/aggregator/config/aggregator.settings.yml

core/modules/aggregator/config/schema/aggregator.schema.yml

...ules/aggregator/config/views.view.aggregator_rss_feed.yml

...or/lib/Drupal/aggregator/Access/CategoriesAccessCheck.php

...gator/lib/Drupal/aggregator/CategoryStorageController.php

.../Drupal/aggregator/CategoryStorageControllerInterface.php

...lib/Drupal/aggregator/Controller/AggregatorController.php

.../modules/aggregator/lib/Drupal/aggregator/Entity/Feed.php

.../modules/aggregator/lib/Drupal/aggregator/Entity/Item.php

...s/aggregator/lib/Drupal/aggregator/FeedFormController.php

...ggregator/lib/Drupal/aggregator/FeedStorageController.php

.../lib/Drupal/aggregator/FeedStorageControllerInterface.php