Remove DetachedBufferRPC and BufferChannel

Both of them used to be the PDX backend of DetachedBuffer (now
BufferHubBuffer) and now not needed as we have the hwbinder backend.

Test: buffer_hub-test, BufferHub_test
Bug: 118891412
Change-Id: I000042ee6f23f98fabcdd60eb2aedda928769314
7 files changed