vrf: VRF_DEFAULT must be 0, remove useless code

vrf: VRF_DEFAULT must be 0, remove useless code

Code was added in the past to support a value of VRF_DEFAULT different
from 0. This option was abandoned, the default vrf id is always 0.

Remove this code, this will simplify the code and improve performance
(use a constant value instead of a function that performs tests).

Signed-off-by: Christophe Gouault <christophe.gouault@6wind.com>