go/printer: remove unused comment parameter

Found by github.com/mvdan/unparam.

Change-Id: I5b0c7cfdc1ab4fe0d79ef4c5a31612bbcf2ff3ad
Reviewed-on: https://go-review.googlesource.com/37833Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/go/printer/printer.go

@@ -325,7 +325,7 @@ func (p *printer) writeString(pos token.Position, s string, isLit bool) {
 // after all pending comments, prev is the previous comment in
 // a group of comments (or nil), and tok is the next token.
 //
-func (p *printer) writeCommentPrefix(pos, next token.Position, prev, comment *ast.Comment, tok token.Token) {
+func (p *printer) writeCommentPrefix(pos, next token.Position, prev *ast.Comment, tok token.Token) {
 	if len(p.output) == 0 {
 		// the comment is the first item to be printed - don't write any whitespace
 		return
@@ -733,7 +733,7 @@ func (p *printer) intersperseComments(next token.Position, tok token.Token) (wro
 	var last *ast.Comment
 	for p.commentBefore(next) {
 		for _, c := range p.comment.List {
-			p.writeCommentPrefix(p.posFor(c.Pos()), next, last, c, tok)
+			p.writeCommentPrefix(p.posFor(c.Pos()), next, last, tok)
 			p.writeComment(c)
 			last = c
 		}