Remove DocObjHost since we cannot support it in .NET Core 3.0+

src/Microsoft.DotNet.Wpf/ApiCompat/Baselines/PresentationFramework-Net48.baseline.txt

@@ -9,5 +9,6 @@ CannotRemoveBaseTypeOrInterface : Type 'System.Windows.Controls.PrintDialogExcep
 CannotRemoveBaseTypeOrInterface : Type 'System.Windows.Data.ValueConversionAttribute' does not implement interface 'System.Runtime.InteropServices._Attribute' in the implementation but it does in the contract.
 CannotRemoveBaseTypeOrInterface : Type 'System.Windows.Data.ValueUnavailableException' does not implement interface 'System.Runtime.InteropServices._Exception' in the implementation but it does in the contract.
 CannotRemoveBaseTypeOrInterface : Type 'System.Windows.Documents.TextElementEditingBehaviorAttribute' does not implement interface 'System.Runtime.InteropServices._Attribute' in the implementation but it does in the contract.
+TypesMustExist : Type 'System.Windows.Interop.DocObjHost' does not exist in the implementation but it does exist in the contract.
 CannotRemoveBaseTypeOrInterface : Type 'System.Windows.Markup.XamlParseException' does not implement interface 'System.Runtime.InteropServices._Exception' in the implementation but it does in the contract.
-Total Issues: 11
+Total Issues: 12

src/Microsoft.DotNet.Wpf/cycle-breakers/PresentationFramework/PresentationFramework.cs

@@ -11217,12 +11217,7 @@ public static partial class BrowserInteropHelper
         public static bool IsBrowserHosted { get { throw null; } }
         public static System.Uri Source { get { throw null; } }
     }
-    public sealed partial class DocObjHost : System.MarshalByRefObject, System.IServiceProvider
-    {
-        public DocObjHost() { }
-        public override object InitializeLifetimeService() { throw null; }
-        object System.IServiceProvider.GetService(System.Type serviceType) { throw null; }
-    }
+
     public sealed partial class DynamicScriptObject : System.Dynamic.DynamicObject
     {
         internal DynamicScriptObject() { }

src/Microsoft.DotNet.Wpf/src/PresentationFramework/PresentationFramework.csproj

@@ -1136,7 +1136,6 @@
     <Compile Include="System\Windows\Input\KeyboardNavigation.cs" />
     <Compile Include="System\Windows\Interop\ActiveXHost.cs" />
     <Compile Include="System\Windows\Interop\BrowserInteropHelper.cs" />
-    <Compile Include="System\Windows\Interop\DocobjHost.cs" />
     <Compile Include="System\Windows\Interop\DynamicScriptObject.cs" />
     <Compile Include="System\Windows\Interop\HwndHost.cs" />
     <Compile Include="System\Windows\Interop\IErrorPage.cs" />

src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Application.cs

@@ -2535,8 +2535,8 @@ private void ConfigAppWindowAndRootElement(object root, Uri uri)
             Window w = root as Window;
             if (w == null)
             {
-                //Creates and returns a NavigationWindow for standalone cases
-                //For browser hosted cases, returns the RootBrowserWindow precreated by docobjhost
+                // Creates and returns a NavigationWindow for standalone cases
+                // Browser hosted cases are no longer supported since .NET Core 3.0
                 NavigationWindow appWin = GetAppWindow();
 
                 //Since we cancel PreBPReady event here, the other navigation events won't fire twice.