LONDON (Reuters) - The cancellation of major sporting events and the decimation of the luxury, entertainment and travel industries is delivering a hammer blow to a global advertising industry that was already reeling from years of tech-led turmoil. What should have been a bumper year with the Euro soccer tournament, Tokyo Olympics and U.S. election looks like it could be one of the worst for ad giants WPP, Omnicom, Publicis and IPG as the economy shuts down.Advertising executives told Reuters..