By Kate Abnett and Nina Chestney BRUSSELS/LONDON (Reuters) – The European Union will set out emergency plans on Wednesday to curb gas usage after President Vladimir Putin warned the bloc that Russian supplies sent via the biggest pipeline to Europe, Nord Stream 1, were at risk of being reduced further. Deliveries via the pipeline, which accounts for more than a third of Russian gas exports to the EU, were halted for 10 days for annual maintenance and are due to resume on Thursday. But supplies via the route had already been reduced before the maintenance outage because of a dispute over sancti…