By Huw Jones LONDON (Reuters) – A shift by European Union markets to halve the time it takes to settle stock trades is a matter of when, not if, as the benefits will outweigh the short-term challenges, the bloc’s financial services chief said on Thursday. From May 28, Wall Street will settle stock trades over one business day rather than two to reduce risk in markets, piling pressure on European markets to follow suit. Canada and Mexico are also taking the same step. “The question is no longer if, but how and when it will happen in the EU,” Mairead McGuinness, EU financial services commissione…