mac_dbdma: Remove leftover `dma_memory_unmap` calls

mac_dbdma: Remove leftover `dma_memory_unmap` calls

These were passing a NULL buffer pointer unconditionally, which happens
to behave in a mostly benign way (except for the chance of an excess
memory region unref and a bounce buffer leak). Per the function comment,
this was never meant to be accepted though, and triggers an assertion
with the "softmmu: Support concurrent bounce buffers" change.

Given that the code in question never sets up any mappings, just remove
the unnecessary dma_memory_unmap calls along with the DBDMA_io struct
fields that are now entirely unused.

Signed-off-by: Mattias Nissler <mnissler@rivosinc.com>
Message-Id: <20240916175708.1829059-1-mnissler@rivosinc.com>
Fixes: be1e343995 ("macio: switch over to new byte-aligned DMA helpers")
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Tested-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>