Skip clearing the QIcon cache on QIconLoader theme key change

QThemeIconEngine takes care of creating a new icon engine when
the icon theme key changes, so we don't need to invalidate the
entire QIcon cache.

Change-Id: Ie3bca00a9146d1f429b2a6f8ab0b39e15834d158
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>
(cherry picked from commit 4dacaaf15a51860f3a7a2d72ca033ccc38165de1)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/image/qiconloader.cpp

@@ -137,9 +137,10 @@ void QIconLoader::updateSystemTheme()
 
 void QIconLoader::invalidateKey()
 {
+    // Invalidating the key here will result in QThemeIconEngine
+    // recreating the actual engine the next time the icon is used.
+    // We don't need to clear the QIcon cache itself.
     m_themeKey++;
-
-    QIconPrivate::clearIconCache();
 }
 
 QString QIconLoader::themeName() const