Remove producer_owns_ in producer_channel

producer_owns_ was created before the creation of buffer state atomics.
It serves a similar purpose of atomics that:
1. prevent producer from gaining a buffer that still have pending
consumers.
2. prevent consumer from acquiring a buffer that is still owned by
producer.
Therefore, producer_owns_ is redundant now.

Bug: 119331650
Test: all tests are passing
on Blueline:
AHardwareBufferTest BufferHubBuffer_test BufferHubMetadata_test
buffer_hub_binder_service-test buffer_hub_queue_producer-test
libgui_test libsensor_test vrflinger_test buffer_hub-test
buffer_hub_queue-test dvr_buffer_queue-test dvr_api-test
dvr_display-test

on Vega:
AHardwareBufferTest BufferHubBuffer_test BufferHubMetadata_test
buffer_hub_queue_producer-test  buffer_hub-test buffer_hub_queue-test
dvr_buffer_queue-test dvr_api-test libdvrtracking-test

Change-Id: I4a6b4c2d5c741a2befe1600b607c308f607dcd49