Steward Healthcare System has abruptly resigned from the state’s leading hospital association in the midst of a turbulent debate over the future of health care reform. The for-profit company and its 10 hospitals resigned, effective immediately, from the Massachusetts Hospital Association on Dec. 23, according to an email from MHA president Lynn Nicholas. Steward officials said they quit the association and its consensus views because the company wanted to be a “single, coherent voice’’ on health care reform.