Cleanup unused logic for ProducerBuffer.Detach

ProducerBuffer.Detach is disabled during migration. Therefore, related
pdx rpc code is no longer needed and could be removed. This is also
required to move BufferNode off IonBuffer, as one of the constructor of
BufferNode is called inside these code.

Test: "atest buffer_hub-test" passed.
Bug: 112338294
Change-Id: I950a2d0f59362f217636da9310f7bfff23127092