i40e: Fix receive buffer starvation for AF_XDP

Magnus's fix to resolve a potential receive buffer starvation for AF_XDP
got applied to both the i40e_xsk_umem_enable/disable() functions, when it
should have only been applied to the "enable".  So clean up the undesired
code in the disable function.

CC: Magnus Karlsson <magnus.karlsson@intel.com>
Fixes: 1f459bdc ("i40e: fix potential RX buffer starvation for AF_XDP")
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>