Remove unused fb_cb related fields from VP9_COMMON
Remove unused fb_cb related fields from VP9_COMMON

Remove the cb_priv, get_fb_cb, release_fb_cb, and int_frame_buffers
fields from the VP9_COMMON struct. They are not being used.

Change-Id: I235194aa8b315cd8ec9405bbba5feb3bee69f7e0

webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx