[dev.link] cmd/link: remove unused slow paths from BytesAt/StringAt
[dev.link] cmd/link: remove unused slow paths from BytesAt/StringAt

This change removes the NewReader function (no longer used by objdump)
and prunes away the now unused code paths from Reader.BytesAt and
Reader.StringAt, which helps with performance. At the moment the
reader operates by always ingesting the entire object file (either via
direct read or by mmap), meaning that there will always be a slice
available for us to index into.

Change-Id: I3af7396effe19e50ed594fe8d82fd2d15465687c
Reviewed-on: https://go-review.googlesource.com/c/go/+/201437
Run-TryBot: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Reviewed-by: Jeremy Faller <b3f594e10a9edcf5413cf1190121d45078c62290@golang.org>

cloudflare/go,cloudflare/go,cloudflare/go,golang/go,golang/go,nawawi/go,akutz/go,cloudflare/go,nawawi/go,CAFxX/go,nawawi/go,CAFxX/go,cloudflare/go,akutz/go,CAFxX/go,golang/go,nawawi/go,akutz/go,golang/go,nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,nawawi/go,golang/go,akutz/go,CAFxX/go,nawawi/go,nawawi/go,akutz/go,CAFxX/go,akutz/go,akutz/go,akutz/go,golang/go,cloudflare/go,golang/go