[dev.link] cmd/link: delete sym.Symbol and sym.Reloc

[dev.link] cmd/link: delete sym.Symbol and sym.Reloc

This deletes all sym.Symbol and sym.Reloc references. This is
certainly not complete, and there are more cleanups to do. But I
feel this makes a good first round.

Change-Id: I7621d016957f7ef114be5f0606fcb3ad6aee71c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/234097
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>