By Tom Sims, Lawrence White and John O’Donnell FRANKFURT (Reuters) – Amid fears of a Russian invasion of Ukraine, Italy’s UniCredit has backed out of a potential acquisition in Russia and Austria’s Raiffeisen Bank International has set aside risk provisions for possible sanctions on Russia. What the region’s banks now fear most is that Russia gets banned from a widely used payment system, bankers told Reuters, with one describing such a move as an “atomic bomb” for the industry because it would prevent the repayment of debts. Here’s what is at stake for Europe’s banks as the crisis shows no si…