Remove Platform::prepareShutdown()

Platform::prepareShutdown() was introduced to work around the issue
where the platform accesses destroyed OutputDeviceInterface objects.

Since we no longer query OutputDeviceInterface for output info, the
Platform::prepareShutdown() function can be dropped.

src/main_wayland.cpp

@@ -129,9 +129,6 @@ ApplicationWayland::~ApplicationWayland()
         return;
     }
 
-    if (auto *platform = kwinApp()->platform()) {
-        platform->prepareShutdown();
-    }
     // need to unload all effects prior to destroying X connection as they might do X calls
     if (effects) {
         static_cast<EffectsHandlerImpl*>(effects)->unloadAllEffects();

src/platform.cpp

@@ -91,11 +91,6 @@ XRenderBackend *Platform::createXRenderBackend()
 }
 #endif
 
-void Platform::prepareShutdown()
-{
-    setOutputsEnabled(false);
-}
-
 Edge *Platform::createScreenEdge(ScreenEdges *edges)
 {
     return new Edge(edges);

src/platform.h

@@ -80,13 +80,6 @@ public:
         return nullptr;
     }
 
-    /**
-     * Informs the Platform that it is about to go down and shall do appropriate cleanup.
-     * Child classes can override this function but must call the parent implementation in
-     * the end.
-     */
-    virtual void prepareShutdown();
-
     /**
      * Allows the platform to create a platform specific screen edge.
      * The default implementation creates a Edge.

src/plugins/platforms/drm/drm_backend.cpp

@@ -65,6 +65,7 @@ DrmBackend::DrmBackend(QObject *parent)
 
 DrmBackend::~DrmBackend()
 {
+    writeOutputsConfiguration();
     qDeleteAll(m_gpus);
 }
 
@@ -73,15 +74,6 @@ Session *DrmBackend::session() const
     return m_session;
 }
 
-void DrmBackend::prepareShutdown()
-{
-    writeOutputsConfiguration();
-    for (DrmOutput *output : m_outputs) {
-        output->teardown();
-    }
-    Platform::prepareShutdown();
-}
-
 Outputs DrmBackend::outputs() const
 {
     return m_outputs;

src/plugins/platforms/drm/drm_backend.h

@@ -43,7 +43,6 @@ public:
     DmaBufTexture *createDmaBufTexture(const QSize &size) override;
     Session *session() const override;
     bool initialize() override;
-    void prepareShutdown() override;
 
     Outputs outputs() const override;
     Outputs enabledOutputs() const override;