WARSAW, June 21 (Reuters) - Poland's ruling party leader Jaroslaw Kaczynski returned to the cabinet as deputy prime minister on Wednesday, taking up a more prominent role ahead of a closely fought election expected in October or November. The return of Kaczynski, widely viewed as Poland's de-facto ruler, comes amid a stuttering campaign by the conservative ruling party, Law and Justice (PiS), for a third term that would likely see it continue policies that have set it at odds with the European Union. "Thank you for joining the government, thank you because the previous tasks that you performed had great...