see attached patch
My gut feeling is that this is the wrong place to do it since the user_dashboard table belongs to the Dashboard plugin.
Theoretically, the user_dashboard table may not exist if the Dashboard plugin is uninstalled. (#546)
I think the Piwik convention would be to add a hook, i.e., PostEvent('UsersManager.deleteUser'); and add a listener to Dashboard.php.
(In ) fixes #1304 - delete dashboard layout when deleting user
(In ) Fixes #1304 Piwik_Query accepts bind parameters but Piwik_Exec doesn't, was causing deleting a user to issue an error