Dorchester community leaders are worried about the future of Carney Hospital after the abrupt departure last week of its president, Bill Walczak, a long-time neighborhood activist who was hired by Carney’s new owner only 14 months ago. Walczak took issue with a spokesman for Steward Health Care System, the parent of Carney, who said last Friday that Walczak had resigned from his post. “I did not resign from the Carney Hospital,’’ Walczak said Wednesday. He declined to elaborate on the circumstances behind his leaving the 186-bed hospital.