labels_helper.rb

Since label presenter is used in label index view, label class
check doesn't work as expected because the class is now LabelPresenter.

Also `label.subject` doesn't work as expected now because Label's model
`subject` method is shadowed by Gitlab's presenter's method which uses
`subject` for referencing the original object.

Instead we use a presenter's method for both checks now.

`label_deletion_confirm_text` is not used anywhere so it's removed