By Lucy Raitano (Reuters) – Royal Mail and broadcaster ITV will lose their spots on Britain’s benchmark FTSE 100 equity index after their respective valuations plunged this year. British Gas owner Centrica and UK student housing provider Unite Group will replace them when the next quarterly reshuffle takes effect on June 20. Index provider FTSE Russell last week flagged British chemicals maker Johnson Matthey as a potential addition but it failed to make the cut in the end. Harbour Energy, shares of which fell on Wednesday on talk that it would be demoted from the index, kept its place. Shares…