examples/vhost: remove VLAN strip option

examples/vhost: remove VLAN strip option

When VMDQ is enabled, different NICs have different behaviors for
disabling VLAN strip. In detail, i40e only enables/disables it of
PF's main vsi; fm10k cannot disable VLAN strip, etc. We now remove
this option, --vlan-strip, to reduce any confusion. And now, VLAN
strip will be enabled and cannot be disabled.

Reported-by: Qian Xu <qian.q.xu@intel.com>
Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>