Remove unused css and css class selectors

What does this MR do and why?

Remove unused css

• multiple sidebar classes no longer exist
• remove references to them and the corresponding css
• the css is references in ee_trial_banner.js, which is used in both ee_trial_banner/index.js => _ee_subscribable_banner.html.haml and main_ee.js => _verification_reminder.html.haml, were introduced in Trial and license purchases inside GitLab EE (!2266 - merged) to be used in header/_default.html.haml and layouts/_page.html.haml

Screenshots or screen recordings

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

Screen	Before	After
Verification banner
Subscribable banner

CSS class	global search results
js-navbar-gitlab
js-page-with-sidebar

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/_ee_subscribable_banner.html.haml b/ee/app/views/layouts/header/_ee_subscribable_banner.html.haml
index cf47067ed32c..93ddcc6a3191 100644
--- a/ee/app/views/layouts/header/_ee_subscribable_banner.html.haml
+++ b/ee/app/views/layouts/header/_ee_subscribable_banner.html.haml
@@ -1,5 +1,5 @@
-- subscribable = gitlab_subscription_or_license
-- message = gitlab_subscription_message_or_license_message
+- subscribable = gitlab_subscription_or_license || true
+- message = gitlab_subscription_message_or_license_message || "Important Message"
 - subject = gitlab_subscription_subject_or_license_subject
 
 - if message.present? && subscribable.present?
diff --git a/ee/app/views/layouts/header/_verification_reminder.html.haml b/ee/app/views/layouts/header/_verification_reminder.html.haml
index c68024ad6e7f..d0c08b7eb7dc 100644
--- a/ee/app/views/layouts/header/_verification_reminder.html.haml
+++ b/ee/app/views/layouts/header/_verification_reminder.html.haml
@@ -1,2 +1,2 @@
-- if show_verification_reminder?
+- if show_verification_reminder? || true
   .js-verification-reminder

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)