Deleted the ajax spinner on the states index

There is only 1 observe_field in the Spree admin, found here:
https://github.com/spree/spree/blob/master/backend/app/views/spree/admin/states/index.html.erb#L15

When you change that field the javascript from admin.js will show a loading indicator.
But there is also a global loading indicator, which will be shown on every ajaxStart, shown here:
https://github.com/spree/spree/blob/master/backend/app/assets/javascripts/spree/backend/progress.coffee#L23

At this moment 2 spinners are shown.
So this spinner on the states index could be deprecated, the global spinner will handle it.