Use JavaScript pluralize for budget staff

We were using two different systems to set translations in JavaScript:
to set the text for languages, we were using data attributes, and to set
the text for staff members, we were using AJAX calls.

I find data attributes keep the code more simple, since there's no need
to define an extra route and controller action. Furthermore, the user
experience is better because response times are faster.

So now both places use data attributes.

app/controllers/admin/budgets_controller.rb

@@ -72,13 +72,6 @@ class Admin::BudgetsController < Admin::BaseController
     end
   end
 
-  def assigned_users_translation
-    render json: { administrators: t("admin.budgets.edit.administrators", count: params[:administrators].to_i),
-                   valuators: t("admin.budgets.edit.valuators", count: params[:valuators].to_i),
-                   trackers: t("admin.budgets.edit.trackers", count: params[:trackers].to_i)
-    }
-  end
-
   private
 
     def budget_params