[openvpn] Removed mtu_disc from auto_client method

MTU disc is not supported on most OpenWRT devices tested.

Author: nemesifier

netjsonconfig/backends/openvpn/openvpn.py

@@ -110,7 +110,6 @@ def auto_client(
             'cert',
             'key',
             'pkcs12',
-            'mtu_disc',
             'mtu_test',
             'fragment',
             'mssfix',