Remove obsolete interface.

Remove obsolete interface in SigninManager.

SigninManager.Observer was renamed to SignInFlowObserver, which should now
be used instead.

BUG=421058
Committed: https://crrev.com/1baf0bad7eef2ec6ca75e34babbdefb296722ecd
Cr-Commit-Position: refs/heads/master@{#299191}

[newt (away), 2014-10-10 19:27:48]

newt@chromium.org changed reviewers:
+ nyquist@chromium.org

[newt (away), 2014-10-10 19:27:49]

PTAL

[nyquist, 2014-10-10 19:39:11]

lgtm, but could you expand a little bit on the commit message for new readers
who do not necessarily read or have access to the bug?

when was the interface made obsolete? what should be used instead? why was it
renamed?

[newt (away), 2014-10-10 20:51:21]

The CQ bit was checked by newt@chromium.org

[commit-bot: I haz the power, 2014-10-10 20:53:08]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/648843002/1

[commit-bot: I haz the power, 2014-10-10 21:35:17]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-10 21:35:59]

Patchset 1 (id:??) landed as
https://crrev.com/1baf0bad7eef2ec6ca75e34babbdefb296722ecd
Cr-Commit-Position: refs/heads/master@{#299191}