SAN DIEGO — San Diego took key legal steps Tuesday aimed at reviving efforts to construct the city’s $4 billion Pure Water pipeline, which was abruptly halted this summer by a legal dispute over the use of unionized construction workers. The City Council voted unanimously Tuesday to remove pro-union language from contracts for Pure Water, a recycling system that would purify treated sewage into drinking water and supply one-third of San Diego’s water supply by 2035.