Resolve "Extract EE specific files/lines for Discussion app/serializers"

What does this MR do?

Extracts the EE-specific lines from the IssueEntity, and also removes some duplicate lines. No CE change is needed:

$ git diff app/serializers/issue_entity.rb ~/Code/gitlab-development-kit/gitlab/app/serializers/issue_entity.rb
diff --git a/app/serializers/issue_entity.rb b/Users/seanmcgivern/Code/gitlab-development-kit/gitlab/app/serializers/issue_entity.rb
index 2ff8713efce..16a477c92fa 100644
--- a/app/serializers/issue_entity.rb
+++ b/Users/seanmcgivern/Code/gitlab-development-kit/gitlab/app/serializers/issue_entity.rb
@@ -2,7 +2,6 @@

 class IssueEntity < IssuableEntity
   include TimeTrackableEntity
-  prepend ::EE::IssueEntity

   expose :state
   expose :milestone_id

Does this MR meet the acceptance criteria?

• Conforms to the code review guidelines
  • Has been reviewed by a Backend maintainer
• EE specific content should be in the top level /ee folder

What are the relevant issue numbers?

Closes #6650 (closed).