By Elizabeth Piper KYIV (Reuters) – Ukraine’s richest man has pledged to help rebuild the besieged city of Mariupol, a place close to his heart where he owns two vast steelworks that he says will once again compete globally. Rinat Akhmetov has seen his business empire shattered by eight years of fighting in Ukraine’s east but remains defiant, sure that what he calls “our brave soldiers” will defend the Sea of Azov city reduced to a wasteland by seven weeks of bombardment. For now, though, his Metinvest company, Ukraine’s biggest steelmaker, has announced it cannot deliver its supply contracts …