C#: Cleanup stub project references.

Author: michaelnebel

csharp/ql/test/resources/stubs/ServiceStack.Common/6.2.0/ServiceStack.Common.csproj

@@ -9,7 +9,6 @@
   <ItemGroup>
     <ProjectReference Include="../../ServiceStack.Interfaces/6.2.0/ServiceStack.Interfaces.csproj" />
     <ProjectReference Include="../../ServiceStack.Text/6.2.0/ServiceStack.Text.csproj" />
-    <ProjectReference Include="../../System.Memory/4.5.4/System.Memory.csproj" />
     <ProjectReference Include="../../_frameworks/Microsoft.NETCore.App/Microsoft.NETCore.App.csproj" />
   </ItemGroup>
 </Project>

csharp/ql/test/resources/stubs/ServiceStack.OrmLite/6.2.0/ServiceStack.OrmLite.csproj

@@ -8,7 +8,6 @@
 
   <ItemGroup>
     <ProjectReference Include="../../ServiceStack.Common/6.2.0/ServiceStack.Common.csproj" />
-    <ProjectReference Include="../../System.Memory/4.5.4/System.Memory.csproj" />
     <ProjectReference Include="../../_frameworks/Microsoft.NETCore.App/Microsoft.NETCore.App.csproj" />
   </ItemGroup>
 </Project>

csharp/ql/test/resources/stubs/ServiceStack.Text/6.2.0/ServiceStack.Text.csproj

@@ -7,9 +7,6 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <ProjectReference Include="../../Microsoft.CSharp/4.7.0/Microsoft.CSharp.csproj" />
-    <ProjectReference Include="../../System.Diagnostics.DiagnosticSource/6.0.0/System.Diagnostics.DiagnosticSource.csproj" />
-    <ProjectReference Include="../../System.Memory/4.5.4/System.Memory.csproj" />
     <ProjectReference Include="../../_frameworks/Microsoft.NETCore.App/Microsoft.NETCore.App.csproj" />
   </ItemGroup>
 </Project>