Remove obsolete model usage

We'd rather keep the table so future data migrations work smoothly, so
we change the migration in order to create the translation table without
using models.

db/migrate/20180323190027_add_translate_milestones.rb

@@ -1,15 +1,12 @@
 class AddTranslateMilestones < ActiveRecord::Migration
-  def self.up
-    Budget::Investment::Milestone.create_translation_table!(
-      {
-        title:       :string,
-        description: :text
-      },
-      { migrate_data: true }
-    )
-  end
+  def change
+    create_table :budget_investment_milestone_translations do |t|
+      t.integer :budget_investment_milestone_id, null: false
+      t.string  :locale,                         null: false
+      t.string  :title
+      t.text    :description
 
-  def self.down
-    Budget::Investment::Milestone.drop_translation_table!
+      t.timestamps null: false
+    end
   end
 end

db/schema.rb

@@ -163,15 +163,12 @@ ActiveRecord::Schema.define(version: 20181016204729) do
   create_table "budget_investment_milestone_translations", force: :cascade do |t|
     t.integer  "budget_investment_milestone_id", null: false
     t.string   "locale",                         null: false
-    t.datetime "created_at",                     null: false
-    t.datetime "updated_at",                     null: false
     t.string   "title"
     t.text     "description"
+    t.datetime "created_at",                     null: false
+    t.datetime "updated_at",                     null: false
   end
 
-  add_index "budget_investment_milestone_translations", ["budget_investment_milestone_id"], name: "index_6770e7675fe296cf87aa0fd90492c141b5269e0b", using: :btree
-  add_index "budget_investment_milestone_translations", ["locale"], name: "index_budget_investment_milestone_translations_on_locale", using: :btree
-
   create_table "budget_investment_milestones", force: :cascade do |t|
     t.integer  "investment_id"
     t.string   "title",            limit: 80