Pet products retailer Chewy is backing away from plans for a $135 million fulfillment center in Windsor. Even so, the developers that lined up to erect the giant e-commerce company’s planned 750,000-square-foot building said they are going to proceed with construction anyway. Winstanley and ABB Group — owner of the former Combustion Engineering nuclear-boiler production and testing facilities that once occupied the Great Pond site — are the master developers of the 625-acre Great Pond district, which is targeted for large-scale residential and industrial development. Windsor Town Planner Eric Barz said the development still needs site plan approval. He anticipates design changes based on Chewy’s withdrawal.