Kernel/Graphics: Remove GenericGraphicsAdapter::vga_compatible method

There's no point in keeping this method as we don't really care if a
graphics adapter is VGA compatible or not because we don't use this
method anymore.

Kernel/Graphics/VirtIOGPU/GraphicsAdapter.h

@@ -39,9 +39,6 @@ class VirtIOGraphicsAdapter final
 public:
     static NonnullRefPtr<VirtIOGraphicsAdapter> initialize(PCI::DeviceIdentifier const&);
 
-    // FIXME: There's a VirtIO VGA GPU variant, so we should consider that
-    virtual bool vga_compatible() const override { return false; }
-
     virtual void initialize() override;
     void initialize_3d_device();