Submitting the dashboard configuration form prints "The block settings have been updated" twice

If you go to admin/structure/dashboard and submit the form, you get that message twice (and the form submit handler actually runs twice).

The reason is that the dashboard module and block module share this form via hook_forms(). Previously, submit handlers were not automatically shared via that method, so the dashboard.module explicitly added the submit handler it needed. A recent core change, though, made it so the submit handler is inherited automatically, and therefore all we need to do to fix this is remove that code from the dashboard module, as in the attached patch.