A 50-year-old now-defunct mercury mine remediated with natural vegetation in the late 1990s is no longer leaking toxic metal into Tomales Bay, officials say. The flow of mercury into the bay from the 14-acre mining site during rainy months became a major environmental concern after state researchers measured unusually high quantities of it in area tributaries in the spring of 1998. Dyan Whyte, an assistant executive officer with the state Regional Water Quality Control Board…