cmd/internal/obj: eliminate unnecessary ctxt.Cursym assignment

None of the following code uses it.

Passes toolstash-check -all.

Updates #15756

Change-Id: Ieeaaca8ba31e5c345c0c8a758d520b24be88e173
Reviewed-on: https://go-review.googlesource.com/38444
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>

src/cmd/internal/obj/pass.go

@@ -142,8 +142,6 @@ func linkpatch(ctxt *Link, sym *LSym) {
 	var name string
 	var q *Prog
 
-	ctxt.Cursym = sym
-
 	for p := sym.Text; p != nil; p = p.Link {
 		checkaddr(ctxt, p, &p.From)
 		if p.From3 != nil {