Remove unlimited members during trial alert banner

As mart of Banner cleanup - Removals after onboarding & ne... (gitlab-org#20607) • Marcel van Remmerden, Paul W • 18.9

This MR Removes the "Get the most out of your trial with space for more members" banner and all related code.

Changes

Deleted files:

• ee/app/components/namespaces/free_user_cap/unlimited_members_during_trial_alert_component.rb
• ee/app/components/namespaces/free_user_cap/unlimited_members_during_trial_alert_component.html.haml
• ee/app/views/shared/_unlimited_members_during_trial_alert.html.haml
• ee/spec/components/namespaces/free_user_cap/unlimited_members_during_trial_alert_component_spec.rb
• ee/spec/features/groups/user_interacts_with_unlimited_members_during_trial_alert_spec.rb
• ee/spec/features/projects/user_interacts_with_unlimited_members_during_trial_alert_spec.rb
• ee/spec/support/shared_examples/features/unlimited_members_during_trial_alert_shared_examples.rb

Modified files:

• app/models/users/group_callout.rb - Marked enum value 18 as removed
• app/views/layouts/project.html.haml - Removed render call for the alert
• app/views/layouts/group.html.haml - Removed render call for the alert
• ee/spec/features/learn_gitlab_spec.rb - Removed tests for the alert