Update `epics_controller.rb` to prevent unnecessary Epics query

Summary

With removal of feature flag vue_epics_list happening in !79404 (merged), we no longer need to query @epics within ee/app/controllers/groups/epics_controller.rb since the epics list is now rendered using GraphQL instead of Rails, so we can prevent unnecessary DB query.

UPDATE: loading of epics was removed from HTML response, but it's still needed/used in JSON response. We can remove it from JSON response when #351563 (comment 830439858) is rolled-out.