sync: remove Chrome OS specific logic to deal with flimflam shutdown / sync race.

No longer necessary as the ProfileSyncService now aborts sync network traffic on shutdown.

BUG=chromium-os:20841


Review URL: http://codereview.chromium.org/9358007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@120912 0039d316-1c4b-4281-b951-d872f2087c98