dmaengine: ioatdma: remove unused function missed during dma_v2 removal

dmaengine: ioatdma: remove unused function missed during dma_v2 removal

Commit f158697780a2 ("dmaengine: ioatdma: remove ioatdma v2 registration")
missed to remove dca2_tag_map_valid() during its removal. Hence, since
then, dca2_tag_map_valid() is unused and make CC=clang W=1 warns:

  drivers/dma/ioat/dca.c:44:19:
    warning: unused function 'dca2_tag_map_valid' [-Wunused-function]

So, remove this unused function and get rid of a -Wused-function warning.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Link: https://lore.kernel.org/r/20201113081248.26416-1-lukas.bulwahn@gmail.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>