Problem/Motivation
In #2691675: Replace deprecated entityManager() in ControllerBase descendents, we noticed that NodePreviewController::title() is not called once in our tests, despite actually being defined as the title_callback.
It's not called because the parent controller sets #title directly, and we never try to get the title for that route from elsewhere (e.g. breadcrumb). And it woud most likely not work anyway as it relies on being able to dynamically load the node through the query argument and then state.
Proposed resolution
Deprecate the untested code, replace it with a static _title: "Node preview" or so title as it doesn't really matter anyway. Maybe even remove completely, not sure what our policy on that one is.