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