GPU: remove unused member from FrameBuffer

Accidentally added in rB2510bd3a5f35d14f5e0e098c79a776916d273223

source/blender/gpu/intern/gpu_framebuffer_private.hh

@@ -94,11 +94,6 @@ class FrameBuffer {
   void **py_ref = nullptr;
 #endif
 
- public:
-  /* Reference of a pointer that needs to be cleaned when deallocating the frame-buffer.
-   * Points to #BPyGPUFrameBuffer::fb */
-  void **ref = nullptr;
-
  public:
   FrameBuffer(const char *name);
   virtual ~FrameBuffer();