Remove GraphicBuffer accessor from DetachedBuffer

Those accessor are not used outside of buffer_hub-test. Moreover, in
the new design, GraphicBuffer will depend on DetachedBuffer rather
than DetachedBuffer depending on GraphicBuffer.

Bug: 112010261
Bug: 112940221
Test: atest buffer_hub-test
Change-Id: I78b5ea8b195b8502fc8bdcda48ca600f6408897a
3 files changed