entities: remove :description from MergeRequest

It is already part of ProjectEntity.

CHANGELOG

@@ -63,6 +63,7 @@ v 8.10.0 (unreleased)
   - Added setting to set new users by default as external !4545 (Dravere)
   - Add min value for project limit field on user's form !3622 (jastkand)
   - Add reminder to not paste private SSH keys !4399 (Ingo Blechschmidt)
+  - Remove duplicate `description` field in `MergeRequest` entities (Ben Boeckel)
 v 8.9.5
   - Add more debug info to import/export and memory killer. !5108

lib/api/entities.rb

@@ -199,7 +199,6 @@ module API
       expose :author, :assignee, using: Entities::UserBasic
       expose :source_project_id, :target_project_id
       expose :label_names, as: :labels
-      expose :description
       expose :work_in_progress?, as: :work_in_progress
       expose :milestone, using: Entities::Milestone
       expose :merge_when_build_succeeds