zebra: Bug fixes in vtysh doc string, mcast group handling and vni deletion handling...

zebra: Bug fixes in vtysh doc string, mcast group handling and vni deletion handling with single vxlan device

This patch addresses following bug fixes

- Fix vtysh doc string in "show evpn access-vlan..." command
- Multicast group handling was little complex. This change avoids calling
  multiple functions and directly calls the zebra_vxlan_if_update_vni for
  mcast group updates.
- When a vlan-vni map is removed, the removed vni deletion was happening
  in FRR with SVD config. This was resulting in stale vni and not
  resulting propagation of the vni deletion.
  During vni cleanup (zebra_vxlan_if_vni_clean) zebra_vxlan_if_vni_del
  was called for vni delete which is not correct. We should be calling
  zebra_vxlan_if_vni_entry_del for the given vni entry.

Signed-off-by: Sharath Ramamurthy <sramamurthy@nvidia.com>