The Join test was doing something remarkable and unnecessary instead of
just using ... on a slice. Maybe it was an editing relic.

Fix it by deleting the monstrosity.

Change-Id: I5b90c6d539d334a9c27e57d26dacd831721cfcfe
Reviewed-on: https://go-review.googlesource.com/20727Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>