$form = \Fieldset::instance('complex_form');
$form->remove('field_name');
Currently I do :
// Update existent fieldset
$user_form = \Fieldset::instance('user_form');
$user_form
->add('new_password', ...)
->add('repeat_new_password', ...);
// Remove creation "password repeat" field
$update_form = $user_form->field();
unset($update_form['password_repeat']);
$view->form = $update_form;
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