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Celtics owner Wyc Grousbeck doesn’t regret his “fireworks” comment from March, even though the Celtics’ offseason has been a dud thus far. Grousbeck isn’t backing down after his initial prediction for the Celtics’ improvement hasn’t come true so far. He is bullish on his young coach, Brad Stevens, disdainful of tanking talk for 2014-15, and unregretful about linking the parquet and pyrotechnics with a quote that will live on in the Boston sports lexicon — and may come back to haunt him.