Remove progress and completion shown on Get Started/Learn GitLab

What does this MR do and why?

For Learn Gitlab and Get Started:

1. remove percentage pill from the navigation item in the sidebar
2. remove progress indicator
3. remove icons that indicate the item completion
4. stop making items disabled when they are completed

Related to #562174 (closed)

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

Click to expand

1. Setup gdk to simulate SaaS - https://docs.gitlab.com/ee/development/ee_features.html#simulate-a-saas-instance and restart GDK

2. Create a new group and project.

3. In rails console, onboard that group that was just created

   ::Onboarding::Progress.onboard(Group.last)

4. In the gitlab admin UI, edit the group to be on an ultimate trial plan(edit button from http://gdk.test:3000/admin/groups)

5. In rails console(bundle exec rails c), change the trial, trial start and end date for the group

   Group.last.gitlab_subscription.update_columns(trial_starts_on: Date.today, trial_ends_on: 30.days.from_now, trial: true)

6. Go to the get started path for the group/project: For instance, http://gdk.test:3000/logan225-group/logan225-project/-/get_started

7. To view learn gitlab, go to http://gdk.test:3000/logan225-group/logan225-project/-/learn_gitlab. This is a shortcut, the nav item text will still be "Get started".

MR acceptance checklist

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