remove rails-observers

Rails yanked out observers many many years ago, instead the functionality
was yanked out to a gem that is very lightly maintained.

For example: if we want to upgrade to rails 5 there is no published gem

Internally the usage of observers had quite a few problem.

The series of refactors renamed a bunch of classes to give us more clarity
and removed some magic.

Gemfile

@@ -9,7 +9,6 @@ end
 if rails_master?
   gem 'arel', git: 'https://github.com/rails/arel.git'
   gem 'rails', git: 'https://github.com/rails/rails.git'
-  gem 'rails-observers', git: 'https://github.com/rails/rails-observers.git'
   gem 'seed-fu', git: 'https://github.com/SamSaffron/seed-fu.git', branch: 'discourse'
 else
   # Rails 5 is going to ship with Action Cable, we have no use for it as
@@ -29,8 +28,6 @@ else
   # gem 'railties'
   # gem 'sprockets-rails'
   gem 'rails', '~> 4.2'
-
-  gem 'rails-observers'
   gem 'seed-fu', '~> 2.3.5'
 end