BOSTON — It wasn’t the easiest of nights for the Celtics, but they overwhelmed the Sixers down the stretch in a 117-99 win Tuesday at TD Garden. The C’s have now won a season-best nine straight games as they improved to 46-12 on the season. The Sixers dropped to 33-25 as they look to stay afloat in the East standings. While the Sixers are without the reigning MVP in Joel Embiid, they stuck…