Revert "KCM: Workaround unknown Qt issue that causes the revert dialog to be invisible"

This reverts commit ace23390.

The thing this was working around was fixed in
systemsettings@1022d0b0,
so we don't need it anymore.

kcm/kcm.cpp

@@ -173,9 +173,7 @@ void KCMKScreen::doSave(bool force)
         m_configHandler->updateInitialData();
 
         if (!m_settingsReverted && m_configHandler->shouldTestNewSettings()) {
-            QTimer::singleShot(100, this, [this]() {
-                Q_EMIT showRevertWarning();
-            });
+            Q_EMIT showRevertWarning();
         } else {
             m_settingsReverted = false;
             m_stopUpdatesFromBackend = false;