DORTMUND, Germany — Borussia Dortmund fans protested their club’s controversial sponsorship deal with an arms manufacturer during the team’s Bundesliga opener on Saturday. The second half of Dortmund’s 2-0 win over Eintracht Frankfurt started against a backdrop of banners criticizing Dortmund’s three-year sponsorship deal with Düsseldorf-based Rheinmetall – the world’s largest maker of artillery ammunition. On Wednesday, supporter association Südtribüne …