dmaengine: remove DMA unmap from drivers

Remove support for DMA unmapping from drivers as it is no longer
needed (DMA core code is now handling it).

Cc: Vinod Koul <vinod.koul@intel.com>
Cc: Tomasz Figa <t.figa@samsung.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
[djbw: fix up chan2parent() unused warning in drivers/dma/dw/core.c]
Signed-off-by: Dan Williams <dan.j.williams@intel.com>