By Andrea Mandala and Valentina Za MILAN (Reuters) -Retail investors dumped the rights to buy into a 2 billion euro ($2 billion) cash call at Italian energy services group Saipem on Tuesday, heightening risks for banks that agreed to mop up any unsold shares unless institutional buyers pick up the slack. Saipem is tapping investors in difficult markets after a shock profit downgrade in January driven by worsening margins on contracts. The capital increase runs until July 11. Joint global coordinators BNP Paribas, Citi, Deutsche Bank, HSBC, Intesa Sanpaolo, UniCredit and joint bookrunners ABN A…