Remove unused Net.IPv6ConnectDuration histogram.

Remove unused Net.IPv6ConnectDuration histogram.

This is one of the top 20 histogram by used data
size, but is unowned (thus doesn't show up on
the dashboard by default) and the data for it
has 99% of of samples fall under 3ms.

BUG=387734
Committed: https://crrev.com/eae926da123e398cce19490095714560a53934c0
Cr-Commit-Position: refs/heads/master@{#310170}

[Alexei Svitkine (slow), 2015-01-06 21:18:32]

asvitkine@chromium.org changed reviewers:
+ mmenke@chromium.org

[mmenke, 2015-01-06 21:19:46]

LGTM!  Thanks for doing this!

[Alexei Svitkine (slow), 2015-01-06 21:23:11]

The CQ bit was checked by asvitkine@chromium.org

[commit-bot: I haz the power, 2015-01-06 21:23:58]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/836233002/1

[commit-bot: I haz the power, 2015-01-06 23:33:18]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-06 23:34:20]

Patchset 1 (id:??) landed as
https://crrev.com/eae926da123e398cce19490095714560a53934c0
Cr-Commit-Position: refs/heads/master@{#310170}