cmd/internal/obj: eliminate stray ctxt.Cursym write

It is explicitly assigned in each of the
assemblers as needed.
I plan to remove Cursym entirely eventually,
but this is a helpful intermediate step.

Passes toolstash-check -all.

Updates #15756

Change-Id: Id7ddefae2def439af44d03053886ca8cc935731f
Reviewed-on: https://go-review.googlesource.com/38727


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>