[SyncFS] Drop task cancellation of closure style task from SyncTaskManager

This CL drops task cancellation for tasks that scheduled by SyncTaskManager::ScheduleTask.
Existing caller of the function doesn't require the cancellation for this layer.

BUG=344769

Review URL: https://codereview.chromium.org/193523005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256243 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256243