Drop Platform::requiresCompositing()

It doesn't seem to be used elsewhere other than
Options::isUseCompositing(), which is quite X11-specific.

src/backends/x11/standalone/x11_standalone_platform.cpp

@@ -200,11 +200,6 @@ void X11StandalonePlatform::createPlatformCursor(QObject *parent)
 #endif
 }
 
-bool X11StandalonePlatform::requiresCompositing() const
-{
-    return false;
-}
-
 bool X11StandalonePlatform::openGLCompositingIsBroken() const
 {
     auto timestamp = KConfigGroup(kwinApp()->config(), "Compositing").readEntry(QLatin1String("LastFailureTimestamp"), 0);

src/backends/x11/standalone/x11_standalone_platform.h

@@ -40,7 +40,6 @@ public:
     std::unique_ptr<OpenGLBackend> createOpenGLBackend() override;
     std::unique_ptr<Edge> createScreenEdge(ScreenEdges *parent) override;
     void createPlatformCursor(QObject *parent = nullptr) override;
-    bool requiresCompositing() const override;
     bool compositingPossible() const override;
     QString compositingNotPossibleReason() const override;
     bool openGLCompositingIsBroken() const override;

src/core/platform.cpp

@@ -152,11 +152,6 @@ void Platform::setSceneEglDisplay(EGLDisplay display)
     m_eglDisplay = display;
 }
 
-bool Platform::requiresCompositing() const
-{
-    return true;
-}
-
 bool Platform::compositingPossible() const
 {
     return true;

src/core/platform.h

@@ -97,12 +97,6 @@ public:
      */
     void setSceneEglGlobalShareContext(EGLContext context);
 
-    /**
-     * Whether the Platform requires compositing for rendering.
-     * Default implementation returns @c true. If the implementing Platform allows to be used
-     * without compositing (e.g. rendering is done by the windowing system), re-implement this method.
-     */
-    virtual bool requiresCompositing() const;
     /**
      * Whether Compositing is possible in the Platform.
      * Returning @c false in this method makes only sense if requiresCompositing returns @c false.

src/dbusinterface.cpp

@@ -327,7 +327,7 @@ bool CompositorDBusInterface::isOpenGLBroken() const
 
 bool CompositorDBusInterface::platformRequiresCompositing() const
 {
-    return kwinApp()->platform()->requiresCompositing();
+    return kwinApp()->operationMode() != Application::OperationModeX11; // TODO: Remove this property?
 }
 
 void CompositorDBusInterface::resume()

src/options.cpp

@@ -1086,7 +1086,7 @@ QStringList Options::modifierOnlyDBusShortcut(Qt::KeyboardModifier mod) const
 
 bool Options::isUseCompositing() const
 {
-    return m_useCompositing || kwinApp()->platform()->requiresCompositing();
+    return m_useCompositing;
 }
 
 } // namespace