You could do it with a Regex:
(\<!--)(.*?)-\>)
Read your file into a string, regex it, and write it back:
string data = File.ReadAllText(path);
string noComments = Regex.Replace(data, @"(\<-)(.*?)-\>)", "");
File.WriteAlText(path, noComments);
But I have no idea why you would want to remove comments: they are generally there for a very good reason...-->