Move unneeded ReactNativeHost out of ReactWindows-Desktop

`//vnect/Shared/Shared.vcxitems` is included by two project:

1. `//vnext/Microsoft.ReactNative/Microsoft.ReactNative.vcxproj`
2. `//vnext/Desktop/React.Windows.Desktop.vcxproj`

(2) is a static library that doesn’t require `ReactNativeHost`
compilation units.  Drop them as having them leads to compiler errors
due to `ReactNativeHost.cpp` including `XamlApplication.h` which in
turn includes `Xaml.XamlApplication.g.*` which requires
`XamlApplication.idl` to be generating them.  This won’t happen due to
the project type of (2).

Author: sundaramramaswamy

vnext/Shared/Shared.vcxitems

@@ -263,6 +263,7 @@
       <DependentUpon>.$(MSBuildThisFileDirectory)..\Microsoft.ReactNative\IReactPackageBuilder.idl</DependentUpon>
     </ClCompile>
     <ClCompile Include="$(MSBuildThisFileDirectory)..\Microsoft.ReactNative\ReactNativeHost.cpp">
+      <ExcludedFromBuild Condition="'$(UseFabric)' != 'true' OR '$(SupportWinUI3Islands)' != 'true'">true</ExcludedFromBuild>
       <DependentUpon>$(MSBuildThisFileDirectory)..\Microsoft.ReactNative\ReactNativeHost.idl</DependentUpon>
     </ClCompile>
     <ClCompile Include="$(MSBuildThisFileDirectory)..\Microsoft.ReactNative\RedBoxHandler.cpp">
@@ -580,11 +581,11 @@
     <Midl Include="$(MSBuildThisFileDirectory)..\Microsoft.ReactNative\QuirkSettings.idl" />
     <Midl Include="$(MSBuildThisFileDirectory)..\Microsoft.ReactNative\ReactCoreInjection.idl" />
     <Midl Include="$(MSBuildThisFileDirectory)..\Microsoft.ReactNative\ReactInstanceSettings.idl" />
-    <Midl Include="$(MSBuildThisFileDirectory)..\Microsoft.ReactNative\ReactNativeHost.idl" />
     <Midl Include="$(MSBuildThisFileDirectory)..\Microsoft.ReactNative\RedBoxHandler.idl" />
     <Midl Include="$(MSBuildThisFileDirectory)..\Microsoft.ReactNative\Timer.idl" />
   </ItemGroup>
   <ItemGroup Condition="'$(UseFabric)' == 'true' OR '$(IncludeFabricInterface)' == 'true'">
+    <Midl Condition="'$(UseFabric)' == 'true' OR '$(IncludeFabricInterface)' == 'true'" Include="$(MSBuildThisFileDirectory)..\Microsoft.ReactNative\ReactNativeHost.idl" />
     <Midl Condition="'$(UseFabric)' == 'true' OR '$(IncludeFabricInterface)' == 'true'" Include="$(ReactNativeWindowsDir)Microsoft.ReactNative\ComponentView.idl" />
     <Midl Condition="'$(UseFabric)' == 'true' OR '$(IncludeFabricInterface)' == 'true'" Include="$(ReactNativeWindowsDir)Microsoft.ReactNative\Composition.Input.idl" />
     <Midl Condition="'$(UseFabric)' == 'true' OR '$(IncludeFabricInterface)' == 'true'" Include="$(ReactNativeWindowsDir)Microsoft.ReactNative\CompositionComponentView.idl" />