Remove references to stale winmd files to fix incremental builds (#1404)

The original attempt at a fix for this was too aggressive:
https://github.com/microsoft/cppwinrt/pull/1381/files

This fix is targeted specifically at removing references to stale winmd files that have yet to be copied from referenced projects (e.g., from a runtime component to an app).

nuget/Microsoft.Windows.CppWinRT.targets

@@ -861,7 +861,9 @@ $(XamlMetaDataProviderPch)
  Condition="'@(Page)@(ApplicationDefinition)' != '' and '$(XamlLanguage)' == 'CppWinRT'"
  DependsOnTargets="$(CppWinRTAddXamlReferencesDependsOn);CppWinRTGetResolvedWinMD;GetCppWinRTProjectWinMDReferences">
  <ItemGroup>
- <XamlReferencesToCompile Include="@(WinMDFullPath);@(CppWinRTDynamicProjectWinMDReferences)" />
+ <XamlReferencesToCompile Include="$(OutDir)*.winmd" />
+ <!--Remove references to stale artifacts that will be copied later-->
+ <XamlReferencesToCompile Remove="@(ReferenceCopyLocalPaths)" MatchOnMetadata="FileName;Extension"/>
  </ItemGroup>
  </Target>