sfc: Remove unused definitions of EF10 user-mode DMA descriptors

These DMA descriptor types will only be used by the userland
networking stack.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Shradha Shah <sshah@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>