Don't prefix range comments with line number twice

Review emails currently show the line number of the first line of
a range comment twice:

	Line 136: Line 136: Plugin Name
		: ~~~~~~~~~~~~~

Drop the first line number prefix.

Change-Id: I4e1ba22282326bf63c887482ad72d4d5a87df884

gerrit-server/src/main/java/com/google/gerrit/server/mail/CommentSender.java

@@ -170,7 +170,6 @@ public class CommentSender extends ReplyToChangeSender {
     CommentRange range = comment.getRange();
     if (range != null) {
       String prefix = String.format("Line %d: ", range.getStartLine());
-      out.append(prefix);
       for (int n = range.getStartLine(); n <= range.getEndLine(); n++) {
         out.append(n == range.getStartLine()
             ? prefix