remove unused helper

Author: silverwind

web_src/css/helpers.css

@@ -75,7 +75,6 @@ Gitea's private styles use `g-` prefix.
 .gt-self-center { align-self: center !important; }
 .gt-self-start { align-self: flex-start !important; }
 .gt-self-end { align-self: flex-end !important; }
-.gt-box-content { box-sizing: content-box !important; }
 
 .gt-overflow-x-auto { overflow-x: auto !important; }
 .gt-overflow-x-scroll { overflow-x: scroll !important; }