Remove unnecessary SanitizeHTML from code (#29575)

* "mail/issue/default.tmpl": the body is rendered by backend
`markdown.RenderString() HTML`, it has been already sanitized
* "repo/settings/webhook/base_list.tmpl": "Description" is prepared by
backend `ctx.Tr`, it doesn't need to be sanitized

(cherry picked from commit dae7f1ebdbe19620f40e110b285f7c0ecd0bb33b)

docs/content/administration/mail-templates.en-us.md

@@ -224,7 +224,7 @@ Please check [Gitea's logs](administration/logging-config.md) for error messages
         {{if not (eq .Body "")}}
             <h3>Message content</h3>
             <hr>
-            {{.Body | SanitizeHTML}}
+            {{.Body}}
         {{end}}
     </p>
     <hr>

docs/content/administration/mail-templates.zh-cn.md

@@ -207,7 +207,7 @@ _主题_ 和 _邮件正文_ 由 [Golang的模板引擎](https://go.dev/pkg/text/
         {{if not (eq .Body "")}}
             <h3>消息内容：</h3>
             <hr>
-            {{.Body | SanitizeHTML}}
+            {{.Body}}
         {{end}}
     </p>
     <hr>