By Jesús Aguado and Inti Landauro MADRID (Reuters) -Santander has dropped out of the process to buy Citigroup’s Mexican retail bank after having previously submitted a non-binding offer earlier this year. The euro zone’s second-biggest bank by market value has been expanding in emerging economies in search of faster growth than in its core European markets, where it has been cutting costs to cope with ultra low interest rates. “In relation to the potential sale of certain parts of (Citigroup’s) operations in Mexico, Santander confirms that, after submitting a non-binding offer for the business…