[dhcp-relay] Fix dhcp6relay counter issue (#2866)

Why I did
While deleting a Vlan, clear dhcpv6_relay counter info state_db before dhcp_relay container restart would cause that counter info still exist in state_db, which is incorrect.
Microsoft ADO number: 24211173

How I did it
Clear counter info in state_db after container restart.

How to verify it
Previous ut and build image to verify.

Signed-off-by: Yaqiang Zhu <yaqiangzhu@microsoft.com>

config/vlan.py

@@ -118,15 +118,14 @@ def del_vlan(db, vid, no_restart_dhcp_relay):
     # set dhcpv4_relay table
     set_dhcp_relay_table('VLAN', config_db, vlan, None)
 
-    delete_state_db_entry(vlan)
-
     if not no_restart_dhcp_relay and is_dhcpv6_relay_config_exist(db, vlan):
         # set dhcpv6_relay table
         set_dhcp_relay_table('DHCP_RELAY', config_db, vlan, None)
         # We need to restart dhcp_relay service after dhcpv6_relay config change
         if is_dhcp_relay_running():
             dhcp_relay_util.handle_restart_dhcp_relay_service()
-
+    delete_state_db_entry(vlan)
+
     vlans = db.cfgdb.get_keys('VLAN')
     if not vlans:
         docker_exec_cmd = "docker exec -i swss {}"