i40e: drop misleading function comments

i40e_cleanup_headers has a statement about check against skb being
linear or not which is not relevant anymore, so let's remove it.

Same case for i40e_can_reuse_rx_page, it references things that are not
present there anymore.

Reviewed-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Tested-by: Tony Brelinski <tonyx.brelinski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>