Remove delivery_mode header from amqp 1.0 shovel.

This header is not present if delivery_mode = 2, it does not really
makes sense to leave it when delivery_mode = 1.
The durability of the message is reflected in the durable flag of the
amqp 1.0 message.

deps/rabbitmq_shovel/src/rabbit_amqp10_shovel.erl

@@ -393,6 +393,8 @@ set_message_properties(Props, Msg) ->
                 #{content_encoding => to_binary(Ct)}, M);
          (delivery_mode, 2, M) ->
               amqp10_msg:set_headers(#{durable => true}, M);
+        (delivery_mode, 1, M) ->
+              M;
          (priority, P, M) when is_integer(P) ->
                 amqp10_msg:set_headers(#{priority => P}, M);
          (correlation_id, Ct, M) ->