[dev.link] cmd/link: remove sym.Symbol based host object support
We have converted all the host object readers to the new loader
based mechanism, and no longer do sym.Symbol host object loading.
Remove that support. So we don't have to take care of that in
future changes to the loader.
TODO: there are still code outside the loader than can be
removed. This CL only focuses on the loader.
Change-Id: I67dd75f631964921a7c7a6576ed95b071241484a
Reviewed-on: https://go-review.googlesource.com/c/go/+/217518 Reviewed-by: Than McIntosh <thanm@google.com> Reviewed-by: Jeremy Faller <jeremy@golang.org>