Remove unneeded package references

Author: bording

LibGit2Sharp/LibGit2Sharp.csproj

@@ -33,12 +33,6 @@
     <DotNetCliToolReference Include="dotnet-sourcelink-git" Version="2.1.2" />
   </ItemGroup>
 
-  <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard1.3' ">
-    <PackageReference Include="System.Security.SecureString" Version="4.0.0" />
-    <PackageReference Include="System.IO.UnmanagedMemoryStream" Version="4.0.1" />
-    <PackageReference Include="System.Diagnostics.TraceSource" Version="4.0.0" />
-  </ItemGroup>
-
   <Import Project="CodeGenerator.targets" />
 
   <Target Name="SetNuSpecProperties" BeforeTargets="GenerateNuspec" DependsOnTargets="GetBuildVersion">