Remove not existing feature flag from tests

Tests for `EE::SubscribableBannerHelper` were testing against feature
flag that do not exists (`subscribable_banner`). Remove this flag from
tests, and add some tests for when `subscribable_subscription_banner` is
enabled/disabled.

See https://gitlab.com/gitlab-org/gitlab/-/issues/295308.

ee/spec/helpers/ee/subscribable_banner_helper_spec.rb

@@ -16,13 +16,8 @@ RSpec.describe EE::SubscribableBannerHelper do
       end
     end
 
-    context 'when feature flag is enabled' do
     let(:license) { double(:license) }
 
-      before do
-        stub_feature_flags(subscribable_banner: true)
-      end
-
     context 'when instance variable true' do
       before do
         assign(:display_subscription_banner, true)
@@ -91,16 +86,15 @@ RSpec.describe EE::SubscribableBannerHelper do
       end
     end
   end
-  end
 
   describe '#gitlab_subscription_message_or_license_message' do
     subject { helper.gitlab_subscription_message_or_license_message }
 
     let(:message) { double(:message) }
 
-    context 'when feature flag is enabled' do
+    context 'when subscribable_subscription_banner feature flag is enabled' do
       before do
-        stub_feature_flags(subscribable_banner: true)
+        stub_feature_flags(subscribable_subscription_banner: true)
       end
 
       context 'when instance variable true' do
@@ -201,6 +195,19 @@ RSpec.describe EE::SubscribableBannerHelper do
         end
       end
     end
+
+    context 'when subscribable_subscription_banner feature flag is disabled' do
+      before do
+        stub_feature_flags(subscribable_subscription_banner: false)
+        assign(:display_subscription_banner, true)
+        allow(::Gitlab::CurrentSettings).to receive(:should_check_namespace_plan?).and_return(true)
+      end
+
+      it 'returns the license message' do
+        expect(helper).to receive(:license_message).and_return(message)
+        expect(subject).to eq(message)
+      end
+    end
   end
 
   describe '#display_subscription_banner!' do