ES: Remove hack for STI bug on Snippet subclass

Related spec is passing in:
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13917/
because now the configuration is stored in the centralized `Config`,
which is then used during initialization of `ApplicationClassProxy`.
No dynamic inheritance is involved, so bug is bypassed.

app/models/personal_snippet.rb

@@ -3,5 +3,3 @@
 class PersonalSnippet < Snippet
   include WithUploads
 end
-
-PersonalSnippet.prepend_if_ee('EE::PersonalSnippet')

app/models/project_snippet.rb

@@ -12,5 +12,3 @@ class ProjectSnippet < Snippet
   participant :author
   participant :notes_with_associations
 end
-
-ProjectSnippet.prepend_if_ee('EE::ProjectSnippet')

ee/app/models/ee/personal_snippet.rb

-# frozen_string_literal: true
-
-module EE
-  module PersonalSnippet
-    extend ActiveSupport::Concern
-
-    prepended do
-      include Elastic::SnippetsSearch
-    end
-  end
-end

ee/app/models/ee/project_snippet.rb

-# frozen_string_literal: true
-
-module EE
-  module ProjectSnippet
-    extend ActiveSupport::Concern
-
-    prepended do
-      include Elastic::SnippetsSearch
-    end
-  end
-end