cmd/internal/ld: remove some dead code

cmd/internal/ld: remove some dead code

Just little bits and pieces I noticed were unused in passing, and
some more found with https://github.com/opennota/check.

Change-Id: I199fecdbf8dc2ff9076cf4ea81395275c7f171c3
Reviewed-on: https://go-review.googlesource.com/7033
Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>