fix: #3319318 Private fields not shown to users with "View webform submissions for own node" permission

Vladimir Roudakovrequested to merge
issue/webform-3319318:3319318-private-fields-3x into 6.3.x

Issue #1458540 by larskleiner: Change unset($form_state['rebuild']) to $form_state['rebuild'] = FALSE to prevent notices.

Closes #3319318

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