Issue #2224581 by alexpott, larowlan, jhodgdon, mgifford: Delete forum data on uninstall.

core/config/schema/core.data_types.schema.yml

...lib/Drupal/Core/Config/Entity/ConfigDependencyManager.php

core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php

core/modules/block/src/Tests/BlockConfigSchemaTest.php

core/modules/comment/src/Tests/CommentFieldsTest.php

core/modules/config/src/Tests/ConfigDependencyTest.php

core/modules/config/src/Tests/ConfigImportAllTest.php

core/modules/config/src/Tests/ConfigImportUITest.php

core/modules/config/src/Tests/ConfigImporterTest.php

...ig/tests/config_test/config/schema/config_test.schema.yml

...odules/config/tests/config_test/src/Entity/ConfigTest.php

core/modules/forum/config/install/node.type.forum.yml

core/modules/forum/forum.module

core/modules/forum/src/Tests/ForumUninstallTest.php

core/modules/node/src/Tests/NodeTypePersistenceTest.php

core/modules/node/src/Tests/NodeTypeTest.php

core/modules/system/src/Tests/Module/DependencyTest.php

.../modules/system/src/Tests/Module/InstallUninstallTest.php

...pal/Tests/Core/Config/Entity/ConfigEntityBaseUnitTest.php