[dev.link] cmd/link: remove more of unused sym.Symbol based DWARF code

[dev.link] cmd/link: remove more of unused sym.Symbol based DWARF code

Remove another set of unused functions, no longer needed now that
loader.Sym based DWARF type generation is the default.

Change-Id: Ie3e63551d75acf559c550768ba6c95fe4d6e67ba
Reviewed-on: https://go-review.googlesource.com/c/go/+/222021
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>