async_tx: kill needless module_{init|exit}

If module_init and module_exit are nops then neither need to be defined.

[ Impact: pure cleanup ]

Reviewed-by: Andre Noll <maan@systemlinux.org>
Acked-by: Maciej Sosnowski <maciej.sosnowski@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>