remove sign_in_form_vue feature flag

Peter Hegmanrequested to merge
remove-sign_in_form_vue-ff into master

What does this MR do and why?

Related to #588330 (closed)

In gitlab-org#19615 (closed) we migrated the sign in form to Vue. This has been enabled on gitlab.com and default enabled in %18.9. We left the feature flag in the codebase to de-risk the change. This MR cleans up the feature flag, old code, and corresponding tests after it has been relased for one milestone (%18.10)

Tip for reviewing

Because we are unwrapping specs from the with_and_without_sign_in_form_vue block there are a lot of whitespace changes. If you Ignore whitespace changes it may be easier to review.

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Edited by Peter Hegman

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