A quick pass through link.go, mostly removing fields that are not
used on the "creating a single object file" side of the fence.

Change-Id: I35ba41378c2c418f7df2f2f88dce65bc64a1a45d
Reviewed-on: https://go-review.googlesource.com/7672
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>