Remove stamper from content model (AlchemyCMS#1808)

The content model always ever is created within an element and
it does not make sense to have this information twice.

app/models/alchemy/content.rb

@@ -28,8 +28,6 @@ class Content < BaseRecord
     belongs_to :element, touch: true, inverse_of: :contents
     has_one :page, through: :element
 
-    stampable stamper_class_name: Alchemy.user_class_name
-
     # Essence scopes
     scope :essence_booleans, -> { where(essence_type: "Alchemy::EssenceBoolean") }
     scope :essence_dates, -> { where(essence_type: "Alchemy::EssenceDate") }

db/migrate/20200226213334_alchemy_four_point_four.rb

@@ -24,8 +24,6 @@ def up
         t.references "element", null: false
         t.datetime "created_at", precision: 6, null: false
         t.datetime "updated_at", precision: 6, null: false
-        t.references "creator"
-        t.references "updater"
       end
     end
 

spec/dummy/db/schema.rb

@@ -34,12 +34,8 @@
     t.integer "element_id", null: false
     t.datetime "created_at", precision: 6, null: false
     t.datetime "updated_at", precision: 6, null: false
-    t.integer "creator_id"
-    t.integer "updater_id"
-    t.index ["creator_id"], name: "index_alchemy_contents_on_creator_id"
     t.index ["element_id"], name: "index_alchemy_contents_on_element_id"
     t.index ["essence_type", "essence_id"], name: "index_alchemy_contents_on_essence_type_and_essence_id", unique: true
-    t.index ["updater_id"], name: "index_alchemy_contents_on_updater_id"
   end
 
   create_table "alchemy_elements", force: :cascade do |t|