[dev.link] cmd/link: remove the second result of MakeSymbolUpdater

[dev.link] cmd/link: remove the second result of MakeSymbolUpdater

With unique global indices, MakeSymbolUpdater will not change the
symbol's index. So no need to return a new index.

Change-Id: I5b4fd6a0167cc74476880bbf4382c524ecde7721
Reviewed-on: https://go-review.googlesource.com/c/go/+/219227
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>