cmd/compile: remove -dolinkobj flag

This used to be used by cmd/vet and some assembly generation tests, but
those were removed in CL 37691 and CL 107336. No point in keeping an
unneeded flag around.

Fixes #28220.

Change-Id: I59f8546954ab36ea61ceba81c10d6e16d74b966a
Reviewed-on: https://go-review.googlesource.com/c/142677


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>