Remove FALLBACK_HOST reference from introdialog.cpp (fix Qt build)

The clearnet fallback host was removed from bootstrap.h in the previous
commit but introdialog.cpp still referenced Bootstrap::FALLBACK_HOST.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/qt/introdialog.cpp

@@ -292,12 +292,6 @@ bool IntroDialog::pickDataDirectory()
             };
 
             bool success = Bootstrap::DownloadBootstrap(host, dataDirPath, progressFn, strError);
-            if (!success) {
-                host = Bootstrap::FALLBACK_HOST;
-                progress.setValue(0);
-                success = Bootstrap::DownloadBootstrap(host, dataDirPath, progressFn, strError);
-            }
-
             if (!success) {
                 QMessageBox::warning(0, "Triangles",
                     QString("Could not download blockchain snapshot:\n%1\n\n"