Refactor PhabricatorBadgesEditRecipientsController to remove dead code

Summary: $form in $dialog = id(new AphrontDialogView())->appendForm($form) is only defined when if ($can_edit) was true beforehand. But that was always true. Thus add a variable definition (and remove some unused variables like $form_box).

Test Plan:
Visit the page /badges/recipients/1/ or also directly /badges/recipients/1/add/ and add some recipients. It still works as before.

Visit the page without enough permissions. It does not allow to edit them, as before.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25744