This internally exported function allowed merging environment variable
lists, and was mostly a convenience for the rest of cmd/go/internal.
It seems to date all the way back to 2013.

However, since CL 37586 in early 2017, os/exec has already taken care of
deduplicating environment variable lists. Thus, it's unnecessary for
cmd/go to take care of that before calling exec.Cmd.Start.

Moreover, because os/exec will deduplicate the list in any case, we're
adding extra work in all these scenarios.

Finally, remove an unnecessary addition of GOROOT= in internal/tool.
cfg.OrigEnv may not have the correct GOROOT set up, but os.Environ does;
cmd/go's main function makes sure of that.

Change-Id: I1f92f65fb927dc15bc7b0397cfd1a572b6337bb3
Reviewed-on: https://go-review.googlesource.com/c/go/+/164703
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>