By Pratima Desai LONDON (Reuters) – The London Metal Exchange faces a struggle to regain its dominant position in global nickel trading as volumes slide and participants flee an increasingly volatile market in the wake of trade mayhem earlier this year. Nickel volumes on the world’s oldest and largest venue for trading metals collapsed after the LME suspended its contract for a week and cancelled all trades on March 8, when prices doubled in a few hours to a record above $100,000 a tonne. LME data shows many participants have abandoned the nickel market, a trend several traders say looks set t…