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/qt/bitcoingui.cpp

@@ -1618,8 +1618,8 @@ void BitcoinGUI::subscribeToCoreSignals()
     m_handler_message_box = m_node.handleMessageBox([this](const bilingual_str& message, const std::string& caption, unsigned int style) {
         return ThreadSafeMessageBox(this, message, caption, style);
     });
-    m_handler_question = m_node.handleQuestion([this](const bilingual_str& message, const std::string& /*non_interactive_message*/, const std::string& caption, unsigned int style) {
-        return ThreadSafeMessageBox(this, message, caption, style);
+    m_handler_question = m_node.handleQuestion([this](const bilingual_str& message, const std::string& /*non_interactive_message*/, unsigned int style) {
+        return ThreadSafeMessageBox(this, message, /*caption=*/"", style);
     });
 }