cmd/internal/obj: more idiomatic object writer

Change-Id: I41722ee605ea76a6b52e8a7e1e10f2293cef1a7a
Reviewed-on: https://go-review.googlesource.com/21371Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/internal/obj/link.go

@@ -658,7 +658,6 @@ type Link struct {
 	Textp         *LSym
 	Etextp        *LSym
 	Errors        int
-	RefsWritten   int // Number of symbol references already written to object file.
 
 	// state for writing objects
 	Text []*LSym