dmaengine: Remove net_dma leftovers

Commit 7bce d397 510a ("net_dma: simple removal") removed the net_dma support
entirely but left some functions and prototypes in the dmaengine header.
Remove them.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>