Cleanup test code for symbolic links.

Change-Id: I25f561cd34dc4d120a4143f933619d233a6cffc5
Reviewed-on: https://go-review.googlesource.com/27573Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>