refactor: Remove empty caption from ThreadSafeMessageBox

The caption was empty for all call-sites, so this refactor does not
change any behavior.

Note that noui_ThreadSafeMessageBoxRedirect is test-only, so no end-user
behavior is changed here.

src/qt/bitcoingui.cpp

@@ -1615,8 +1615,8 @@ static bool ThreadSafeMessageBox(BitcoinGUI* gui, const bilingual_str& message,
 void BitcoinGUI::subscribeToCoreSignals()
 {
     // Connect signals to client
-    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_message_box = m_node.handleMessageBox([this](const bilingual_str& message, 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);