refactor: update registration css and html

Update `Registration.css` and `Registration.html` to enhance UI design. Added a new css class
`.quiqqer-frontendUsers-controls-registrars-list` with properties for flex display, direction, gap,
and width. This class was applied in `Registration.html` to improve the layout of `registrars`
section. Also, removed an unnecessary line in `Registration.php`.