Revert Pajamas migration in shared/_signup_omniauth_provider_list.haml

What does this MR do and why?

Revert "Merge branch 'pj-migration-day-omniauth-button-sjpark' into 'master'"

This reverts merge request !126557 (merged)

This resolves #420803 (closed). The Pajamas buttonComponent used renders an anchor, which currently does not trigger snowplow click event tracking when it's doing POST. This is also why in the original implementation that added the snowplow tracking, the author has to use button_to instead of link_to, see the discussion about this.

To get tracking back, we decided to revert the migration and follow-up with more investigation on how to make Pajamas buttonComponent compatible with snowplow tracking.

How to set up and validate locally

1. Setup your local GDK to simulate a SAAS instance
2. Logout
3. Navigate to http://gdk.test:3000/users/sign_up
4. Clicking SSO button should generate a tracking event with free_registration label in the snowplow debugger extension like so:

MR acceptance checklist

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