By MacDonald Dzirutwe and Libby George LAGOS/LONDON (Reuters) – Shell’s exit from Nigeria’s onshore oil sector highlights risks oil majors face in Africa’s biggest exporter but has raised hopes that local firms could reverse the output decline from the Niger Delta, industry officials and analysts said. Shell – which pioneered Nigeria’s oil industry – is the most prominent Western company to exit the Delta, a region blighted by pollution, oil theft and pipeline vandalism. Those issues have for years stymied investment – and throttled production and government finances. The company’s sale of its…