Remove duplicate submission filter from the 5 branch

The duplicate submission filter introduced with http://drupal.org/node/107358 causes more trouble then it solves IMO.

See http://drupal.org/node/141606 for an issue with views, but undocumented cases I've encountered were with Comment mover, Form store and User badges. In all cases, form submission doesn't result in a call to the submit handlers upon subsequent submissions of a certain form. This usually depends on the data submitted.

An example: when you upload an image with User badges there's no problem. However, you cannot upload another image unless you visit another form first.

These issues can be remedied by setting the property #skip_duplicate_check to TRUE on the form. However, I expect that many custom or contributed modules will fail in certain ways the moment sites upgrade to a hypothetical 5.2.

I see this as an API change, a big one at that, and therefore not suitable for the 5 branch.