When we display the hint string for e.g. (or swipe finger), the login
box gets pushed up. This is leftover from older styling and should be
removed. The padding-top is unnecessary and can also be removed, so just
remove the whole line styling login-dialog-message-hint from the
stylesheet.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1582>