Backport #30308 by @silverwind
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="720f178b-1b22-48d4-8704-becb8ce66129">
<img width="405" alt="Screenshot 2024-04-06 at 18 00 28"
src="61c0f8ec-34af-46c5-a3fa-7c5c4d30c7d2">
Co-authored-by: silverwind <me@silverwind.io>
(cherry picked from commit d26ec5f2eb74a6437f998ab0af2ae058a64e06a2)
deeb268e5d