By Tom Balmforth and Max Hunder KYIV (Reuters) – President Volodymyr Zelenskiy sidelined his childhood friend as head of Ukraine’s security service and another close ally as top prosecutor in Kyiv’s biggest internal purge of the war, citing their failure to root out Russian spies. The careers of SBU security service chief Ivan Bakanov and Prosecutor General Iryna Venediktova had personified Zelenskiy’s policy of putting young loyalists in charge of fighting corruption since the former TV comic came to power in 2019. But nearly five months after Russia’s invasion, the president acknowledged tha…