Fix broken BufferHubQueueTest.TestRemoveBuffer

Under certain condition, it's unsafe to nullify the consumer channel
within producer channel's destructor.

Bug: 38137191
Bug: 70046255
Bug: 70912269
Test: buffer_hub-test, buffer_hub_queue-tests, buffer_hub_queue_producer-tesst
Change-Id: Ifbed85e687e71340f6876e4e85c792ae1d6b06f6