Stop the clearContentsTimer when removing an image backing.

It creates a race condition that can lead to crashes when the WebView's
visible property is set to false and the image backing is removed
shortly after.

Task-number: QTBUG-38841
Change-Id: I0775ba33fda9cee5f86ef50a4fbe048602564296
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>