Duplicate drafts are created by auto-save when page cache is enabled

When the feature to auto-save forms that fail validation as a draft was introduced, there was code added to create the draft and remove the form and form_state from the cache. This forces the form to be rebuilt, honoring the new draft submission's sid. Alas, there was code copied from form.inc which checks to see if the page caching is disabled before deleting the form from the cache. This check is not appropriate in the case of webform.

The symptom is lots of duplicate submissions, created in rapid succession as the user tried to submit the form with one or more validation errors.