Remove unused frame buffer callback related fields

The cb_priv, get_fb_cb, release_fb_cb, and int_frame_buffers fields of
the AV1_COMMON struct are unused. They are in the BufferPool struct.

Change-Id: I4112107e9430f3e882847cf4c2eb0004f1d48503
diff --git a/av1/common/onyxc_int.h b/av1/common/onyxc_int.h
index 032aa3a..5442638 100644
--- a/av1/common/onyxc_int.h
+++ b/av1/common/onyxc_int.h
@@ -473,14 +473,6 @@
   int skip_loop_filter;
   int skip_film_grain;
 
-  // Private data associated with the frame buffer callbacks.
-  void *cb_priv;
-  aom_get_frame_buffer_cb_fn_t get_fb_cb;
-  aom_release_frame_buffer_cb_fn_t release_fb_cb;
-
-  // Handles memory for the codec.
-  InternalFrameBufferList int_frame_buffers;
-
   // External BufferPool passed from outside.
   BufferPool *buffer_pool;