refactor: Remove empty caption from ThreadSafeQuestion

There is only one call-site, which provided an empty caption.

Note that noui_ThreadSafeQuestionRedirect is test-only and currently
entrirely unused, so the logging format string change is not a behavior
change.

This refactor does not change any behavior.

src/interfaces/node.h

@@ -225,7 +225,6 @@ public:
     //! Register handler for question messages.
     using QuestionFn = std::function<bool(const bilingual_str& message,
         const std::string& non_interactive_message,
-        const std::string& caption,
         unsigned int style)>;
     virtual std::unique_ptr<Handler> handleQuestion(QuestionFn fn) = 0;