i40e: make functions static and remove dead code

Make local functions static in the file they are used.

Remove functions i40e_aq_add_vlan and i40e_aq_remove_vlan since
they are not used anywhere by current code.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>