Fix checkboxes on mobile view, remove some dead css (#30308) (#30319)

Backport #30308 by @silverwind

Fix the checkbox issues in
#30303 which were existing
problems with these selectors, but made visible with
#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: silverwind <me@silverwind.io>

web_src/css/form.css

@@ -249,21 +249,6 @@ textarea:focus,
   .user.signup form .optional .title {
     margin-left: 250px !important;
   }
-  .user.activate form .inline.field > input,
-  .user.forgot.password form .inline.field > input,
-  .user.reset.password form .inline.field > input,
-  .user.link-account form .inline.field > input,
-  .user.signin form .inline.field > input,
-  .user.signup form .inline.field > input,
-  .user.activate form .inline.field > textarea,
-  .user.forgot.password form .inline.field > textarea,
-  .user.reset.password form .inline.field > textarea,
-  .user.link-account form .inline.field > textarea,
-  .user.signin form .inline.field > textarea,
-  .user.signup form .inline.field > textarea,
-  .oauth-login-link {
-    width: 50%;
-  }
 }
 
 @media (max-width: 767.98px) {
@@ -310,14 +295,7 @@ textarea:focus,
   .user.reset.password form .inline.field > label,
   .user.link-account form .inline.field > label,
   .user.signin form .inline.field > label,
-  .user.signup form .inline.field > label,
-  .user.activate form input,
-  .user.forgot.password form input,
-  .user.reset.password form input,
-  .user.link-account form input,
-  .user.signin form input,
-  .user.signup form input,
-  .oauth-login-link {
+  .user.signup form .inline.field > label {
     width: 100% !important;
   }
 }
@@ -435,9 +413,9 @@ textarea:focus,
   .repository.new.repo form label,
   .repository.new.migrate form label,
   .repository.new.fork form label,
-  .repository.new.repo form input,
-  .repository.new.migrate form input,
-  .repository.new.fork form input,
+  .repository.new.repo form .inline.field > input,
+  .repository.new.migrate form .inline.field > input,
+  .repository.new.fork form .inline.field > input,
   .repository.new.fork form .field a,
   .repository.new.repo form .selection.dropdown,
   .repository.new.migrate form .selection.dropdown,