Remove feature flag check from migration helpers

Remove `database_async_index_creation` feature flag check from
migration helpers related to async index creation.

See https://gitlab.com/gitlab-org/gitlab/-/issues/354776.

lib/gitlab/database/async_indexes/migration_helpers.rb

@@ -72,8 +72,7 @@ module Gitlab
         end
 
         def async_index_creation_available?
-          ApplicationRecord.connection.table_exists?(:postgres_async_indexes) &&
-            Feature.enabled?(:database_async_index_creation, type: :ops)
+          ApplicationRecord.connection.table_exists?(:postgres_async_indexes)
         end
       end
     end

spec/lib/gitlab/database/async_indexes/migration_helpers_spec.rb

@@ -16,11 +16,6 @@ RSpec.describe Gitlab::Database::AsyncIndexes::MigrationHelpers do
   describe '#unprepare_async_index' do
     let!(:async_index) { create(:postgres_async_index, name: index_name) }
 
-    context 'when the flag is enabled' do
-      before do
-        stub_feature_flags(database_async_index_creation: true)
-      end
-
     it 'destroys the record' do
       expect do
         migration.unprepare_async_index(table_name, 'id')
@@ -48,26 +43,10 @@ RSpec.describe Gitlab::Database::AsyncIndexes::MigrationHelpers do
     end
   end
 
-    context 'when the feature flag is disabled' do
-      it 'does not destroy the record' do
-        stub_feature_flags(database_async_index_creation: false)
-
-        expect do
-          migration.unprepare_async_index(table_name, 'id')
-        end.not_to change { index_model.where(name: index_name).count }
-      end
-    end
-  end
-
   describe '#unprepare_async_index_by_name' do
     let(:index_name) { "index_#{table_name}_on_id" }
     let!(:async_index) { create(:postgres_async_index, name: index_name) }
 
-    context 'when the flag is enabled' do
-      before do
-        stub_feature_flags(database_async_index_creation: true)
-      end
-
     it 'destroys the record' do
       expect do
         migration.unprepare_async_index_by_name(table_name, index_name)
@@ -85,27 +64,11 @@ RSpec.describe Gitlab::Database::AsyncIndexes::MigrationHelpers do
     end
   end
 
-    context 'when the feature flag is disabled' do
-      it 'does not destroy the record' do
-        stub_feature_flags(database_async_index_creation: false)
-
-        expect do
-          migration.unprepare_async_index_by_name(table_name, index_name)
-        end.not_to change { index_model.where(name: index_name).count }
-      end
-    end
-  end
-
   describe '#prepare_async_index' do
     before do
       connection.create_table(table_name)
     end
 
-    context 'when the feature flag is enabled' do
-      before do
-        stub_feature_flags(database_async_index_creation: true)
-      end
-
     it 'creates the record for the async index' do
       expect do
         migration.prepare_async_index(table_name, 'id')
@@ -179,15 +142,4 @@ RSpec.describe Gitlab::Database::AsyncIndexes::MigrationHelpers do
       end
     end
   end
-
-    context 'when the feature flag is disabled' do
-      it 'does not create the record' do
-        stub_feature_flags(database_async_index_creation: false)
-
-        expect do
-          migration.prepare_async_index(table_name, 'id')
-        end.not_to change { index_model.where(name: index_name).count }
-      end
-    end
-  end
 end