Improve in_epics scope

Instead of selecting epics issue ids in a subquery,
an innter join is used.

ee/app/models/ee/issue.rb

@@ -23,10 +23,7 @@ module EE
       scope :order_weight_asc, -> { reorder ::Gitlab::Database.nulls_last_order('weight') }
       scope :no_epic, -> { left_outer_joins(:epic_issue).where(epic_issues: { epic_id: nil }) }
       scope :any_epic, -> { joins(:epic_issue) }
-      scope :in_epics, ->(epics) do
-        issue_ids = EpicIssue.where(epic_id: epics).select(:issue_id)
-        id_in(issue_ids)
-      end
+      scope :in_epics, ->(epics) { joins(:epic_issue).where(epic_issues: { epic_id: epics }) }
       scope :no_iteration, -> { where(sprint_id: nil) }
       scope :any_iteration, -> { where.not(sprint_id: nil) }
       scope :in_iterations, ->(iterations) { where(sprint_id: iterations) }