A lack of standardized policies and an increasingly litigious culture have combined to make doctors wary of abandonment claims and medical malpractice lawsuits, according to Boston-area healthcare lawyers. As a result, the lawyers say that in recent years they have fielded more inquiries from doctors and healthcare organizations about the best way to terminate a doctor-patient relationship. While statistics on patient "firings" are difficult to track, Boston practitioners say the numbers are trending upward.