Remove duplicate hidden_field and fix Rubocop

Signed-off-by: Rémy Coutable <remy@rymai.me>

app/views/shared/issuable/_sidebar.html.haml

@@ -93,7 +93,6 @@
               .light None
           - if can?(current_user, :"admin_#{issuable.to_ability_name}", @project)
             .selectbox.hide-collapsed
-              = hidden_field_tag "#{issuable.to_ability_name}[due_date]", issuable.due_date
               = f.hidden_field :due_date, value: issuable.due_date
               .dropdown
                 %button.dropdown-menu-toggle.js-due-date-select{ type: 'button', data: { toggle: 'dropdown', field_name: "#{issuable.to_ability_name}[due_date]", ability_name: issuable.to_ability_name, issue_update: issuable_json_path(issuable) } }