Retire modbus module

Author: PatAltimore

articles/iot-edge/TOC.yml

@@ -183,8 +183,6 @@
         href: how-to-authenticate-downstream-device.md
       - name: Connect a downstream device
         href: how-to-connect-downstream-device.md
-      - name: Modbus protocol gateway
-        href: deploy-modbus-gateway.md
     - name: Gateways for IoT Edge devices
       items:
       - name: Configure gateways for IoT Edge devices