dma-mapping: remove an outdated comment from dma-map-ops.h

The "/* CONFIG_ARCH_HAS_DMA_COHERENCE_H */" was an description about the
ARCH_HAS_DMA_COHERENCE_H configure option, but it has been removed since
the dma_default_coherent variable was lifted from the mips architecture
to the driver core. Therefore it doesn't match any compile guard now.
Just remove it.

Fixes: 6d4e9a8e ("driver core: lift dma_default_coherent into common code")
Signed-off-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Signed-off-by: Christoph Hellwig <hch@lst.de>