[WP][UX] Remove "tabs" from log-in, register, and password reset pages.

A follow-up to https://github.com/backdrop/backdrop-issues/issues/3108

I'd really like to redesign the log-in page to remove the "tabs" entirely. The visual "tabs" are a strange way to navigate these three pages. Plus, they really give away that a site is Drupal/Backdrop.

I almost always remove these tabs in my custom themes, and replace them with a markup element in the form containing links to those pages instead. I'd love to see us do that by default for all Backdrop sites.