Disable remaining reflow traversal pruning for now

We don't reuse Flow objects between reflows, so we have no old values to fall
back to.

I think this used to work because FlowContexts (as they were called then) were
stored in a DOM node's LayoutData and reused.  But it's possible that it never
really worked, and my testing when I landed the restyle damage code was
insufficient (I didn't understand the layout code nearly as well back then).