remove warning of label_text method in wrapper, when replace html label tag to other tag

Sometime we would use other html tag(span ex.) to replace label html tag in label
component in custom wrapper:

```ruby
  config.wrappers :custom_wrapper, tag: 'div', class: 'form-group control-group' do |b|
    b.wrapper tag: 'span', class: "control-label" do |l|
      l.use :label_text
    end
    b.wrapper :controls, tag: 'div', class: 'controls' do |i|
      i.use :input
    end
  end
```

After that [breaking wrappers api change](https://github.com/plataformatec/simple_form/pull/997),
this usage will cause deprecated wraning. This patch solves the problem.