[dev.link] cmd/link: clean up some fields

With the new index mapping , we can clean up some fields.
Loader.max should always be equal to len(loader.objSyms) now. And
for external symbols we now give its "local index" as its index
in the payload array, so Relocs.extIdx is no longer useful.
Delete those fields.

Change-Id: If387ff9201ea0d347b954f651f5d4b4ae74937aa
Reviewed-on: https://go-review.googlesource.com/c/go/+/218478


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>