[websql] Remove support for unscheduling tasks

Instead, have tasks check that the database they're operating on still
is open.

This brings us closer to the WebThread API which does not allow for
unscheduling tasks.

BUG=301514
R=michaeln@chromium.org

Review URL: https://codereview.chromium.org/83383003

git-svn-id: svn://svn.chromium.org/blink/trunk@162615 bbb929c8-8fbe-4397-9dbb-9b2b20218538