[EE] Add users search results to global search

What does this MR do?

EE version of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21197

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer

lib/api/search.rb

@@ -66,14 +66,7 @@ def elasticsearch?
       end
 
       params :scope do |options|
-        scope_entities =
-          if Feature.enabled?(:users_search, default_enabled: true)
-            SCOPE_ENTITY
-          else
-            SCOPE_ENTITY.reject { |key, value| key == :users }
-          end
-
-        values = scope_entities.stringify_keys.slice(*options[:values]).keys
+        values = SCOPE_ENTITY.stringify_keys.slice(*options[:values]).keys
         values += %w[wiki_blobs blobs commits] if Gitlab::CurrentSettings.elasticsearch_search?
 
         requires :scope,