cmd/internal/goobj: minor dead code cleanup
cmd/internal/goobj: minor dead code cleanup

Remove a bit of dead code from the Go object file reader (io.ReaderAt
no longer needed in goobj.Reader).

Change-Id: I04150d37fb90b59c9dbe930878d4dd21cdcd7ca7
Reviewed-on: https://go-review.googlesource.com/c/go/+/357309
Trust: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>
Run-TryBot: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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