ARM/dma-mapping: remove the unused virt_to_dma helper

virt_to_dma was only used by the now removed dmabounce code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Marc Zyngier <maz@kernel.org>