RingBufferConsumer - Remove unnecessary grab

The mGraphicBuffer is filled in already in ConsumerBase::acquireBufferLocked
adding it from the slots is not needed.

Test: atest, presubmit, compiles
Bug: 342197847
Flag: com.android.graphics.libgui.flags.wb_ring_buffer
Change-Id: I0969c02da294fc634eb9917697701f34cfe72ae1