Issue #2056513 by tim.plunkett: Remove PluginUI subsystem, provide block plugins UI as one-off.

core/modules/block/block.module

core/modules/block/block.routing.yml

core/modules/block/block.services.yml

core/modules/block/lib/Drupal/block/BlockFormController.php

...b/Drupal/block/Controller/BlockAutocompleteController.php

...block/lib/Drupal/block/Controller/BlockListController.php

core/modules/block/lib/Drupal/block/Form/PlaceBlocksForm.php

...lock/lib/Drupal/block/Plugin/Derivative/BlockPluginUI.php

.../block/lib/Drupal/block/Plugin/PluginUI/BlockPluginUI.php

...dules/block/lib/Drupal/block/Plugin/Type/BlockManager.php

...odules/block/lib/Drupal/block/Routing/RouteSubscriber.php

...ules/block/lib/Drupal/block/Tests/BlockAdminThemeTest.php

...s/block/lib/Drupal/block/Tests/BlockLibrarySearchTest.php

core/modules/block/lib/Drupal/block/Tests/BlockTest.php

...odules/block/lib/Drupal/block/Tests/BlockTitleXSSTest.php

core/modules/block/lib/Drupal/block/Tests/BlockUiTest.php

...block/lib/Drupal/block/Tests/NonDefaultBlockAdminTest.php

core/modules/menu/lib/Drupal/menu/Tests/MenuTest.php

...s/system/lib/Drupal/system/Access/SystemPluginUiCheck.php

.../system/lib/Drupal/system/Plugin/Type/PluginUIManager.php