We fire a single shot timer to wait for the rumble to complete, so we
can call on_complete. SDL keeps an internal account of how long the
rumble should take and might, at that point, still be convinced the
gamepad should rumble. To prevent this race condition, explicitly cancel
the rumble ourselves.
Hopefully fixes the flakiness of
`Tests/LibWeb/Text/input/GamepadAPI/gamepad-rumble.html`.