Remove unused alert overwrite css

What does this MR do and why?

Remove unused alert overwrite css

• the alert css is standard now and managed by gitlab-ui
• remove dead overwrite css

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Currently (without css applied)	If I update the css to be used (e.g. alert => gl-alert)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Navigate to a project
2. Apply the below patch

diff --git a/ee/app/views/layouts/header/_licensed_user_count_threshold.html.haml b/ee/app/views/layouts/header/_licensed_user_count_threshold.html.haml
index 7c165eb532b9..fbe97f06a131 100644
--- a/ee/app/views/layouts/header/_licensed_user_count_threshold.html.haml
+++ b/ee/app/views/layouts/header/_licensed_user_count_threshold.html.haml
@@ -1,10 +1,10 @@
-- return unless show_active_user_count_threshold_banner?
+- return unless true
 .container-fluid.container-limited.pt-3
   = render Pajamas::AlertComponent.new(title: _('Your instance is approaching its licensed user count'),
     alert_options: { class: 'gitlab-ee-license-banner js-admin-licensed-user-count-threshold', data: { feature_id: Users::CalloutsHelper::ACTIVE_USER_COUNT_THRESHOLD, dismiss_endpoint: callouts_path } },
     close_button_options: { type: 'button', data: { testid: 'gitlab-ee-license-banner-dismiss' } }) do |c|
     - c.with_body do
-      = _('Your instance has %{remaining_user_count} users remaining of the %{total_user_count} included in your subscription. You can add more users than the number included in your license, and we will include the overage in your next bill.') % { remaining_user_count: remaining_user_count, total_user_count: total_user_count }
+      = _('Your instance has %{remaining_user_count} users remaining of the %{total_user_count} included in your subscription. You can add more users than the number included in your license, and we will include the overage in your next bill.') % { remaining_user_count: 0, total_user_count: 1 }
     - c.with_actions do
       = link_button_to _('View users statistics'), admin_users_path, class: 'gl-alert-action', variant: :confirm
       = link_button_to _('Contact support'), EE::CUSTOMER_LICENSE_SUPPORT_URL, rel: 'nofollow', class: 'gl-alert-action', variant: :confirm, category: :secondary

3. Verify

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #366605 (closed)