Disable the inheritance column of services in DisableInvalidServiceTemplates migration

The `unless defined?(Service)` was useless since in production env,
models are eager loaded, thus we wouldn't disable the STI, resulting in
the following error:

The single-table inheritance mechanism failed to locate the subclass:
'GitlabCiService'. This error is raised because the column 'type' is
reserved for storing the class in case of inheritance. Please rename
this column if you didn't intend it to be used for storing the
inheritance class or overwrite Service.inheritance_column to use another
column for that
information./opt/gitlab/embedded/service/gitlab-rails/db/post_migrate/20170211073944_disable_invalid_service_templates.rb:11:in `up'
Signed-off-by: Rémy Coutable <remy@rymai.me>

changelogs/unreleased/28850-fix-broken-migration.yml

+---
+title: Fix broken migration when upgrading straight to 8.17.1
+merge_request: 9613
+author:

db/post_migrate/20170211073944_disable_invalid_service_templates.rb

 class DisableInvalidServiceTemplates < ActiveRecord::Migration
   DOWNTIME = false
 
-  unless defined?(Service)
-    class Service < ActiveRecord::Base
-      self.inheritance_column = nil
-    end
+  class Service < ActiveRecord::Base
+    self.inheritance_column = nil
   end
 
   def up