i40e/i40evf: Remove unused hardware receive descriptor code

The hardware supports a 16 byte descriptor for receive, but the
driver was never using it in production.  There was no performance
benefit to the real driver of 16 byte descriptors, so drop a whole
lot of complexity while getting rid of the code.

Also since the previous patch made us use no-split mode all the
time, drop any support in the driver for any other value in dtype
and assume it is always zero (aka no-split).

Hooray for code removal!

Change-ID: I2257e902e4dad84a07b94db6d2e6f4ce69b27bc0
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>