i40e: Remove unused function i40e_vsi_lookup

The function is not used so there is no need to carry it forward.  I have
plans to add a slightly different function that can be inlined to handle
the same kind of functionality.

Change-ID: Ie2dfcb189dc75e5fbc156bac23003e3b4210ae0f
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>