refactor: 移除预留占位节点

Author: ArgoZhang

src/BootstrapBlazor/Components/TreeView/TreeViewRow.razor

@@ -43,28 +43,6 @@
                                                OnUpdateCallbackAsync="OnUpdateCallbackAsync"></TreeViewToolbarEditButton>
                 }
             }
-
-            @* 鼠标拖到节点中央，表示放置到当前节点内部作为最后一个子节点，显示边框预览 *@
-            @if (_previewChildLast)
-            {
-                <div class="tree-preview-child-last"></div>
-            }
-            @* 展开状态下，拖到当前节点靠下的位置，插入到内部作为第一个子节点，显示线条预览 *@
-            @if (_previewChildFirst)
-            {
-                <div class="tree-preview-child-first">
-                    <div class="tree-preview-circle"></div>
-                    <div class="tree-preview-line"></div>
-                </div>
-            }
-            @* 折叠状态或者没有子节点，拖到当前节点靠下的位置，作为同级节点，显示线条预览 *@
-            @if (_previewBelow)
-            {
-                <div class="tree-preview-below">
-                    <div class="tree-preview-circle"></div>
-                    <div class="tree-preview-line"></div>
-                </div>
-            }
         </DynamicElement>
     </div>
 </div>