cmd/cgo: delete unused variable in log statement

visit is just a func, and there's no formatting
verb for it, and it's on an internal-error path.
It has been thus many years, unchanged and unexecuted.

Change-Id: I4c2e2673ee9996218c24143bcc3be3eb4abdff25
Reviewed-on: https://go-review.googlesource.com/32970
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/cgo/ast.go

@@ -297,7 +297,7 @@ func (f *File) walk(x interface{}, context string, visit func(*File, interface{}
 
 	// everything else just recurs
 	default:
-		error_(token.NoPos, "unexpected type %T in walk", x, visit)
+		error_(token.NoPos, "unexpected type %T in walk", x)
 		panic("unexpected type")
 
 	case nil: