Delete groups form is misleading/incorrect

For users without the 'administer nodes' permission, the og delete group for (as set up in og_node_delete_group_form), is misleading. It provides the option to "Delete all group posts which don't also belong to another group," but no action is taken upon form submission. The form should either not provide the option to users not allowed to execute it, or should correctly execute the action.

The attached patch takes the latter approach, by adding the handler og_node_delete_confirm_submit to the form even when the user doesn't have the 'administer nodes' privilege.