Remove feature flag from invite members banner

The invite members banner is displayed on the
Group Overview page when:
- page viewer can admin group
- group has 1 or fewer members including yourself
- the group was not just created, so there is no
"Group successfully created" flash message also
on the page in that spot.

Changelog: added

app/helpers/groups_helper.rb

@@ -206,7 +206,6 @@ module GroupsHelper
   end
 
   def show_invite_banner?(group)
-    Feature.enabled?(:invite_your_teammates_banner_a, group) &&
     can?(current_user, :admin_group, group) &&
     !just_created? &&
     !multiple_members?(group)

changelogs/unreleased/323106-experiment-cleanup-invite_your_teammates_banner_a.yml

+---
+title: Remove feature flag from invite members banner
+merge_request: 59203
+author:
+type: added

config/feature_flags/development/invite_your_teammates_banner_a.yml

----
-name: invite_your_teammates_banner_a
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/37658
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/231275
-milestone: '13.4'
-type: development
-group: group::expansion
-default_enabled: false

spec/helpers/groups_helper_spec.rb

@@ -427,21 +427,16 @@ RSpec.describe GroupsHelper do
     before do
       allow(helper).to receive(:current_user) { current_user }
       allow(helper).to receive(:can?).with(current_user, :admin_group, group).and_return(can_admin_group)
-      stub_feature_flags(invite_your_teammates_banner_a: feature_enabled_flag)
       users.take(group_members_count).each { |user| group.add_guest(user) }
     end
 
     using RSpec::Parameterized::TableSyntax
 
-    where(:feature_enabled_flag, :can_admin_group, :group_members_count, :expected_result) do
-      true  | true  | 1 | true
-      true  | false | 1 | false
-      false | true  | 1 | false
-      false | false | 1 | false
-      true  | true  | 2 | false
-      true  | false | 2 | false
-      false | true  | 2 | false
-      false | false | 2 | false
+    where(:can_admin_group, :group_members_count, :expected_result) do
+      true  | 1 | true
+      false | 1 | false
+      true  | 2 | false
+      false | 2 | false
     end
 
     with_them do