Issue #2104229 by claudiu.cristea: Deprecate file_usage().

core/includes/file.inc

core/modules/editor/editor.module

...es/editor/lib/Drupal/editor/Tests/EditorFileUsageTest.php

core/modules/file/file.module

core/modules/file/lib/Drupal/file/Entity/File.php

.../modules/file/lib/Drupal/file/FileUsage/FileUsageBase.php

core/modules/file/lib/Drupal/file/Tests/DeleteTest.php

core/modules/file/lib/Drupal/file/Tests/FileListingTest.php

core/modules/file/lib/Drupal/file/Tests/UsageTest.php

core/modules/image/image.module