Cleanup login form css classes

Summary

During review of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31619, we noticed some potentially outdated / unused css classes:

• Usage of both d-xl-block and d-lg-block: d-lg-block should cover sizes lg+
• bottom css class: appears in app/assets/stylesheets/framework/lists.scss and applies to the last li direct descendent in a .hover-list which is not how it is being used here
• move-submit-down class

Improvements

We can reduce the number of unused / outdated css classes used in these files

Involved components

Sign in related haml templates: app/views/devise/sessions