Remove :vue_issuable_epic_sidebar frontend feature flag

This feature flag is not used at all.

ee/app/controllers/groups/epics_controller.rb

@@ -17,10 +17,6 @@ class Groups::EpicsController < Groups::ApplicationController
   before_action :verify_group_bulk_edit_enabled!, only: [:bulk_update]
   after_action :log_epic_show, only: :show
 
-  before_action do
-    push_frontend_feature_flag(:vue_issuable_epic_sidebar, @group)
-  end
-
   feature_category :epics
 
   def new

ee/config/feature_flags/development/vue_issuable_epic_sidebar.yml

----
-name: vue_issuable_epic_sidebar
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/18199
-rollout_issue_url:
-milestone: '12.4'
-type: development
-group: group::product planning
-default_enabled: false

spec/spec_helper.rb

@@ -221,7 +221,6 @@ RSpec.configure do |config|
       # of older sidebar.
       # See https://gitlab.com/groups/gitlab-org/-/epics/1863
       stub_feature_flags(vue_issuable_sidebar: false)
-      stub_feature_flags(vue_issuable_epic_sidebar: false)
 
       # Merge request widget GraphQL requests are disabled in the tests
       # for now whilst we migrate as much as we can over the GraphQL