Remove BufferHubBase::Poll method

because the eventfd should not be polled directly after
Consumer/ProducerBuffer are backed by BufferHubBuffer.
The only current instances of BufferHubBase::Poll being used are in
buffer_hub-test

Bug: None
Test: m, mma, buffer_hub-test

Change-Id: Iac586086aed54510c1a8bae020e5ca24d8fd065e