ZAGREB, Croatia — The Obama administration on Wednesday renounced the proclaimed leaders of the Syrian political opposition and said any group seeking to oust President Bashar al-Assad must reject attempts by extremists to “hijack” a legitimate revolution. Secretary of State Hillary Rodham Clinton said the Syrian National Council, or SNC, should no longer be considered the “visible leader” of the opposition. That made official what has been the increasingly obvious sidelining of an opposition group led mostly by middle-age Syrian expatriates.