Remove feature flag to backfill partitioned table

Remove the backfill_partitioned_audit_events feature flag, which was
default enabled to true, and is no longer needed since the migration has
successfully completed.

config/feature_flags/development/backfill_partitioned_audit_events.yml

----
-name: backfill_partitioned_audit_events
-introduced_by_url: 
-rollout_issue_url: 
-group: 
-type: development
-default_enabled: true

lib/gitlab/database/partitioning_migration_helpers/backfill_partitioned_table.rb

@@ -11,8 +11,6 @@ module Gitlab
         PAUSE_SECONDS = 0.25
 
         def perform(start_id, stop_id, source_table, partitioned_table, source_column)
-          return unless Feature.enabled?(:backfill_partitioned_audit_events, default_enabled: true)
-
           if transaction_open?
             raise "Aborting job to backfill partitioned #{source_table} table! Do not run this job in a transaction block!"
           end

spec/lib/gitlab/database/partitioning_migration_helpers/backfill_partitioned_table_spec.rb

@@ -116,23 +116,6 @@ RSpec.describe Gitlab::Database::PartitioningMigrationHelpers::BackfillPartition
       expect(jobs_updated).to eq(1)
     end
 
-    context 'when the feature flag is disabled' do
-      let(:mock_connection) { double('connection') }
-
-      before do
-        allow(subject).to receive(:connection).and_return(mock_connection)
-        stub_feature_flags(backfill_partitioned_audit_events: false)
-      end
-
-      it 'exits without attempting to copy data' do
-        expect(mock_connection).not_to receive(:execute)
-
-        subject.perform(1, 100, source_table, destination_table, unique_key)
-
-        expect(destination_model.count).to eq(0)
-      end
-    end
-
     context 'when the job is run within an explicit transaction block' do
       let(:mock_connection) { double('connection') }