cmd/internal/obj: remove dead Link fields

These are unused after CLs 39922, 40252, 40370, 40371, and 40372.

Change-Id: I76f9276c581067a8cb555de761550d960f6e39b8
Reviewed-on: https://go-review.googlesource.com/40404
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/internal/obj/link.go

@@ -727,12 +727,8 @@ type Link struct {
 	InlTree       InlTree // global inlining tree used by gc/inl.go
 	Imports       []string
 	Plan9privates *LSym
-	Instoffset    int64
-	Autosize      int32
-	Pc            int64
 	DiagFunc      func(string, ...interface{})
 	DebugInfo     func(fn *LSym, curfn interface{}) []*dwarf.Var // if non-nil, curfn is a *gc.Node
-	Cursym        *LSym
 	Version       int
 	Errors        int