Qt: Fix build without libpcap

Change-Id: I9b9b5de1beb23486ccc000feda54c2b20a33f4ad
Reviewed-on: https://code.wireshark.org/review/23251
Reviewed-by: Stig Bjørlykke <stig@bjorlykke.org>

ui/qt/main_window_slots.cpp

@@ -671,8 +671,6 @@ void MainWindow::captureCapturePrepared(capture_session *) {
 //    /* Don't set up main window for a capture file. */
 //    main_set_for_capture_file(FALSE);
     main_ui_->mainStack->setCurrentWidget(&master_split_);
-#else
-    Q_UNUSED(session)
 #endif // HAVE_LIBPCAP
 }