[dev.link] cmd/internal/goobj2: remove old-style accessors

Now that we switched to new style accessors everywhere, remove
the old ones.

Keep goobj2.Sym, Reloc, and Aux for now, as they are still used
on the writer side.

Change-Id: I6f3f0d812ff591ea6f12cd0155232f160d53d453
Reviewed-on: https://go-review.googlesource.com/c/go/+/226802


Reviewed-by: Than McIntosh <thanm@google.com>