i40e: Drop extra copy of function

i40e_release_rx_desc was in two files, but was only used
and needed in txrx.c.  Get rid of the extra copy.

Change-Id: I86e18239aa03531fc198b6c052847475084a9200
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>