[dev.link] cmd/internal/goobj2: remove "2"

Rename
Sym2 -> Sym
Reloc2 -> Reloc
Aux2 -> Aux

Also the Reader methods.

Change-Id: I49f29e2d1cb480f5309e01d7a74b5e0897d826fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/227900


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>