By Gary McWilliams HOUSTON (Reuters) – Exxon Mobil likely will face new pressure to severe ties with Russia’s largest oil producer, said analysts, after rival BP agreed to unload a Rosneft stake. Russia’s attack on Ukraine has unleashed broad economic and political rebukes and corporate withdrawals by banks, technology and other firms unprecedented in their extent. BP on Sunday said it would take a $25 billion writedown to abandon its Rosneft holdings. Exxon holds a 30% stake, alongside Rosneft, Japan’s SODECO and India’s ONGC Videsh, in Sakhalin Island oil and gas fields in Russia’s Far East….