remove extra package reference

Author: twsouthwick

test/DocumentFormat.OpenXml.Generator.Tests/DocumentFormat.OpenXml.Generator.Tests.csproj

@@ -10,7 +10,6 @@
     <PackageReference Include="NuGet.Packaging" />
     <PackageReference Include="NuGet.Protocol" />
     <PackageReference Include="NuGet.Resolver" />
-    <PackageReference Include="System.Text.Json" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\..\gen\DocumentFormat.OpenXml.Generator\DocumentFormat.OpenXml.Generator.csproj" />