backends/wayland: Remove unused attribute

Makes clang complain

src/backends/wayland/wayland_output.cpp

@@ -43,8 +43,7 @@ using namespace KWayland::Client;
 static const int s_refreshRate = 60000; // TODO: can we get refresh rate data from Wayland host?
 
 WaylandCursor::WaylandCursor(WaylandBackend *backend)
-    : m_backend(backend)
-    , m_surface(backend->display()->compositor()->createSurface())
+    : m_surface(backend->display()->compositor()->createSurface())
 {
 }
 

src/backends/wayland/wayland_output.h

@@ -49,7 +49,6 @@ public:
 private:
     void sync();
 
-    WaylandBackend *const m_backend;
     KWayland::Client::Pointer *m_pointer = nullptr;
     std::unique_ptr<KWayland::Client::Surface> m_surface;
     wl_buffer *m_buffer = nullptr;