[fix 2/3] make search_on_category_select hide checkboxes

Note that we inject the CSS via JavaScript instead of using
css_dependencies because we do not wan the CSS to load
when JavaScript is disabled (since in that case the checkboxes
should be visible).

searx/static/plugins/css/search_on_category_select.css

@@ -0,0 +1 @@
+.category input { display: none; }

searx/static/plugins/js/search_on_category_select.js

@@ -40,3 +40,7 @@ function getHttpRequest() {
     }
     return httpRequest;
 }
+
+const style = document.createElement('style');
+style.textContent = '.category input { display: none; }'
+document.head.append(style);