async_tx: kill ASYNC_TX_ASSUME_COHERENT

Remove the unused ASYNC_TX_ASSUME_COHERENT flag.  Async_tx is
meant to hide the difference between asynchronous hardware and synchronous
software operations, this flag requires clients to understand cache
coherency consequences of the async path.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Haavard Skinnemoen <hskinnemoen@atmel.com>