refactor: 移除 _tab 变量

Author: ArgoZhang

src/BootstrapBlazor/Components/Layout/Layout.razor

@@ -131,7 +131,7 @@
     </main>;
 
     RenderFragment RenderTab =>
-    @<Tab ClickTabToNavigation="ClickTabToNavigation" AdditionalAssemblies="@AdditionalAssemblies" @ref="_tab"
+    @<Tab ClickTabToNavigation="ClickTabToNavigation" AdditionalAssemblies="@AdditionalAssemblies"
           ShowExtendButtons="ShowTabExtendButtons" ShowClose="ShowTabItemClose" AllowDrag="AllowDragTab"        
           DefaultUrl="@TabDefaultUrl" ExcludeUrls="@ExcludeUrls" IsOnlyRenderActiveTab="IsOnlyRenderActiveTab"
           TabStyle="TabStyle" ShowToolbar="@ShowToolbar" ToolbarTemplate="@ToolbarTemplate"

src/BootstrapBlazor/Components/Layout/Layout.razor.cs

@@ -457,7 +457,6 @@ public partial class Layout : IHandlerException, ITabHeader
     private IStringLocalizer<Layout>? Localizer { get; set; }
 
     private bool _init;
-    private Tab? _tab = null;
     private LayoutHeader? _layoutHeader = null;
 
     private ITabHeader? TabHeader => ShowTabInHeader ? this : null;