dmaengine: at_hdmac: remove unsuded atc_cleanup_descriptors()

Since patch 7c407d3e54dcc0c79119553c8d5ef176c1d5bc3a (DMA: AT91:
Get residual bytes in dma buffer), the function
atc_cleanup_descriptors() is not used anymore. We remove it to prevent
warnings.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>