dmaengine: at_hdmac: remove unused function

commit 54f8d501 ('dmaengine: remove DMA unmap from drivers')
refactored some code which resulted in an unused function in the at_hdmac
driver:

drivers/dma/at_hdmac_regs.h:350:23: warning: 'chan2parent' defined but
    not used [-Wunused-function]

Fixes: 54f8d501 ('dmaengine: remove DMA unmap from drivers')
Signed-off-by: Olof Johansson <olof@lixom.net>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Acked-by: Vinod Koul <vinod.koul@Intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>