Remove backward compatible VSA code

This change removes backward compatibility code from 13.9 after
removing a duplicated stage event.

app/models/concerns/analytics/cycle_analytics/stage.rb

@@ -49,14 +49,6 @@ module Analytics
         end
       end
 
-      def start_event_identifier
-        backward_compatible_identifier(:start_event_identifier) || super
-      end
-
-      def end_event_identifier
-        backward_compatible_identifier(:end_event_identifier) || super
-      end
-
       def start_event_label_based?
         start_event_identifier && start_event.label_based?
       end
@@ -136,17 +128,6 @@ module Analytics
           .id_in(label_id)
           .exists?
       end
-
-      # Temporary, will be removed in 13.10
-      def backward_compatible_identifier(attribute_name)
-        removed_identifier = 6 # References IssueFirstMentionedInCommit removed on https://gitlab.com/gitlab-org/gitlab/-/merge_requests/51975
-        replacement_identifier = :issue_first_mentioned_in_commit
-
-        # ActiveRecord returns nil if the column value is not part of the Enum definition
-        if self[attribute_name].nil? && read_attribute_before_type_cast(attribute_name) == removed_identifier
-          replacement_identifier
-        end
-      end
     end
   end
 end

ee/spec/models/analytics/cycle_analytics/group_stage_spec.rb

@@ -33,38 +33,4 @@ RSpec.describe Analytics::CycleAnalytics::GroupStage do
       let(:default_params) { { group: parent } }
     end
   end
-
-  context 'when the event identifier is using the old, recently deduplicated identifier' do
-    let(:group) { create(:group) }
-    let(:value_stream) { create(:cycle_analytics_group_value_stream, group: group) }
-    let(:invalid_identifier) { 6 }
-
-    let(:stage_params) do
-      {
-        name: 'My Stage',
-        parent: group,
-        start_event_identifier: :merge_request_created,
-        end_event_identifier: :merge_request_merged,
-        value_stream: value_stream
-      }
-    end
-
-    let(:stage) { described_class.create!(stage_params) }
-
-    before do
-      # update the columns directly so validations are skipped
-      stage.update_column(:start_event_identifier, invalid_identifier)
-      stage.update_column(:end_event_identifier, invalid_identifier)
-    end
-
-    subject { described_class.find(stage.id) }
-
-    it 'loads the correct start event' do
-      expect(subject.start_event).to be_a_kind_of(Gitlab::Analytics::CycleAnalytics::StageEvents::IssueFirstMentionedInCommit)
-    end
-
-    it 'loads the correct end event' do
-      expect(subject.end_event).to be_a_kind_of(Gitlab::Analytics::CycleAnalytics::StageEvents::IssueFirstMentionedInCommit)
-    end
-  end
 end