Fix checkboxes on mobile view, remove some dead css (#30308)

Fix checkboxes on mobile view, remove some dead css (#30308)

Fix the checkbox issues in
https://github.com/go-gitea/gitea/issues/30303 which were existing
problems with these selectors, but made visible with
https://github.com/go-gitea/gitea/pull/30162.

There is a lot of dead/useless CSS in `form.css`, I only fixed the two
problems and remove CSS that was definitely not in use or needed.

<img width="369" alt="Screenshot 2024-04-06 at 18 00 08"
src="https://github.com/go-gitea/gitea/assets/115237/720f178b-1b22-48d4-8704-becb8ce66129">
<img width="405" alt="Screenshot 2024-04-06 at 18 00 28"
src="https://github.com/go-gitea/gitea/assets/115237/61c0f8ec-34af-46c5-a3fa-7c5c4d30c7d2">

Co-authored-by: Giteabot <teabot@gitea.io>