Revert "Add Deepin to the list of GTK based desktop environments"

This reverts commit 39f0633f0b.

The Deepin desktop environment is developed based on Qt, but some
applications use dialog boxes of GTK, and some applications use dialog
boxes of Qt. This doesn't look good. The initial commit was trying
to unify this by always using GTK dialogs.

But this turned out to not look good, so instead the plan is that
all application dialogs can be based on Qt. Reverting the original
commit as a result, as it is no longer applicable.

Pick-to: 6.2
Change-Id: I2824b7b2f292f3896c9f94ccce6979f77d66578b
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/gui/platform/unix/qgenericunixthemes.cpp

@@ -859,8 +859,7 @@ QStringList QGenericUnixTheme::themeNames()
                              << "UNITY"
                              << "MATE"
                              << "XFCE"
-                             << "LXDE"
-                             << "DEEPIN";
+                             << "LXDE";
         const QList<QByteArray> desktopNames = desktopEnvironment.split(':');
         for (const QByteArray &desktopName : desktopNames) {
             if (desktopEnvironment == "KDE") {