ipv6: simplify log message format

This change replaces the existing log format separated by commas and
spaces with space-separated one.

Also removes unnecessary log output in test.

Change-Id: I7fc5da1b58734a7ee1d06da4e15408bef0162b0b
Reviewed-on: https://go-review.googlesource.com/16323
Reviewed-by: Ian Lance Taylor <iant@golang.org>
6 files changed