Remove commentable functionality from photos

This is not (and as far as I know, was never) used. If we want to make
standalone photos commentable, we can always add it back, but it would
also need to change federation for it to work, because comments support
only posts there. But for now it makes the code cleaner and easier to
remove it.

app/models/photo.rb

@@ -6,7 +6,6 @@
 
 class Photo < ApplicationRecord
   include Diaspora::Federated::Base
-  include Diaspora::Commentable
   include Diaspora::Shareable
 
   # NOTE API V1 to be extracted

db/migrate/20190701234716_photos_remove_comment_count.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+class PhotosRemoveCommentCount < ActiveRecord::Migration[5.1]
+  class Comment < ApplicationRecord
+  end
+
+  def change
+    remove_column :photos, :comments_count, :integer
+
+    reversible do |change|
+      change.up { Comment.where(commentable_type: "Photo").delete_all }
+    end
+  end
+end