[dev.link] cmd/compile, cmd/asm: delete old object file format support

There are more cleanups to do, but I want to keep this CL mostly
a pure deletion.

Change-Id: Icd2ff0a4b648eb4adf3d29386542617e49620818
Reviewed-on: https://go-review.googlesource.com/c/go/+/206398
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/internal/obj/link.go

@@ -652,7 +652,6 @@ type Link struct {
 	Flag_linkshared    bool
 	Flag_optimize      bool
 	Flag_locationlists bool
-	Flag_newobj        bool // use new object file format
 	Bso                *bufio.Writer
 	Pathname           string
 	hashmu             sync.Mutex       // protects hash, funchash