By Ludwig Burger and Thomas Escritt BERLIN (Reuters) – Wintershall Dea AG said on Tuesday Russian oligarch German Khan, who was sanctioned by the European Union, was stepping down from its board, causing further upheaval at the German oil company with several ties to Russia. Wintershall Dea said its business activities would be unaffected by the move but it was examining whether LetterOne’s shareholding would be affected. The oil and gas producer is a joint venture of German chemicals giant BASF and LetterOne, the holding company controlled by Russian oligarch Mikail Fridman, who is also linke…