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/init.cpp

@@ -1377,7 +1377,7 @@ static ChainstateLoadResult InitAndLoadChainstate(
     options.coins_error_cb = [] {
         uiInterface.ThreadSafeMessageBox(
             _("Error reading from database, shutting down."),
-            "", CClientUIInterface::MSG_ERROR);
+            CClientUIInterface::MSG_ERROR);
     };
     uiInterface.InitMessage(_("Loading block index…"));
     auto catch_exceptions = [](auto&& f) -> ChainstateLoadResult {