[REF] web: use bootstrap class instead of css

Small code cleanup with using bootstrap class instead of custom css

task-3439226

Part-of: odoo/odoo#136375

addons/web/static/src/core/utils/nested_sortable.js

@@ -124,7 +124,12 @@ export const useNestedSortable = makeDraggableHook({
             ctx.prevNestX = ctx.pointer.x;
         }
         ctx.current.placeHolder = ctx.current.element.cloneNode(false);
-        ctx.current.placeHolder.classList.add("o_nested_sortable_placeholder");
+        ctx.current.placeHolder.classList.add(
+            "o_nested_sortable_placeholder",
+            "w-100",
+            "d-block",
+            "py-0"
+        );
         addCleanup(() => ctx.current.placeHolder.remove());
     },
 

addons/web/static/src/core/utils/nested_sortable.scss

@@ -1,9 +1,5 @@
 .o_nested_sortable_placeholder {
     background-clip: content-box;
     background-color: deepskyblue;
-    display: block;
     height: 5px;
-    padding-top: 0 !important;
-    padding-bottom: 0 !important;
-    width: 100%;
 }