Refactor setting.Other and remove unused SHOW_FOOTER_BRANDING (#24270)

The `SHOW_FOOTER_BRANDING` came from year 2015, and it seems nobody ever
uses it. It only shows an GitHub icon which seems unrelated to Gitea, it
doesn't do what document says. So, remove it.

## ⚠️ Breaking

Users can now remove the key `[other].SHOW_FOOTER_BRANDING` from their
app.ini.

docs/content/doc/administration/config-cheat-sheet.en-us.md

@@ -1391,7 +1391,6 @@ although Github don't support this form.
 
 ## Other (`other`)
 
-- `SHOW_FOOTER_BRANDING`: **false**: Show Gitea branding in the footer.
 - `SHOW_FOOTER_VERSION`: **true**: Show Gitea and Go version information in the footer.
 - `SHOW_FOOTER_TEMPLATE_LOAD_TIME`: **true**: Show time of template execution in the footer.
 - `ENABLE_SITEMAP`: **true**: Generate sitemap.

docs/content/doc/administration/config-cheat-sheet.zh-cn.md

@@ -483,5 +483,4 @@ PROXY_HOSTS = *.github.com
 
 ## Other (`other`)
 
-- `SHOW_FOOTER_BRANDING`: 为真则在页面底部显示Gitea的字样。
 - `SHOW_FOOTER_VERSION`: 为真则在页面底部显示Gitea的版本。