C2BufferQueueBlockPoolData: remove C2BufferQueueBlockPool::Impl field

Originally, C2BufferQueueBlockPoolData has
C2BufferQueueBlockPool::Impl field for calling IGBP::cancelBuffer() at
destructor if the buffer is still held. However,
C2BufferQueueBlockPoolData already has a IGBP field. This CL removes
the unnecessary C2BufferQueueBlockPool::Impl field.

Bug: 174188958
Test: android.media.cts.MediaCodecPlayerTest#testPlaybackSwitchViews

Change-Id: I6baf5c4a1b9d1c27f95d18817628cd6ffcd35478