core/renderbackend: remove unused parent object

As the render backend is always stored in a unique pointer, using a Qt parent
might even cause crashes

src/core/renderbackend.cpp

@@ -147,11 +147,6 @@ std::chrono::nanoseconds OutputFrame::predictedRenderTime() const
     return m_predictedRenderTime;
 }
 
-RenderBackend::RenderBackend(QObject *parent)
-    : QObject(parent)
-{
-}
-
 OutputLayer *RenderBackend::cursorLayer(Output *output)
 {
     return nullptr;

src/core/renderbackend.h

@@ -119,8 +119,6 @@ class KWIN_EXPORT RenderBackend : public QObject
     Q_OBJECT
 
 public:
-    explicit RenderBackend(QObject *parent = nullptr);
-
     virtual CompositingType compositingType() const = 0;
     virtual OverlayWindow *overlayWindow() const;