Issue #2268761 by tim.plunkett: Remove support for function-based forms.

core/includes/batch.inc

core/lib/Drupal/Core/Controller/FormController.php

core/lib/Drupal/Core/Entity/EntityFormBuilder.php

core/lib/Drupal/Core/Form/FormBuilder.php

core/lib/Drupal/Core/Form/FormBuilderInterface.php

core/modules/field/src/Tests/FieldAttachOtherTest.php

core/modules/field_ui/src/Form/FieldEditForm.php

...modules/system/src/Tests/Form/ElementsTableSelectTest.php

...modules/system/src/Tests/Form/FormDefaultHandlersTest.php

core/modules/system/src/Tests/Form/FormTest.php

core/modules/system/src/Tests/Form/StubForm.php

core/modules/views/includes/ajax.inc

core/modules/views_ui/src/Form/Ajax/ViewsFormBase.php

.../tests/Drupal/Tests/Core/Entity/EntityFormBuilderTest.php

core/tests/Drupal/Tests/Core/Form/FormBuilderTest.php

core/tests/Drupal/Tests/Core/Form/FormTestBase.php