Remove unnecessary System.Collections.Immutable package reference

Author: rameel

Directory.Packages.props

@@ -9,6 +9,5 @@
     <PackageVersion Include="MinVer" Version="6.0.0" />
     <PackageVersion Include="NUnit" Version="4.3.2" />
     <PackageVersion Include="NUnit3TestAdapter" Version="5.0.0" />
-    <PackageVersion Include="System.Collections.Immutable" Version="6.0.0" />
   </ItemGroup>
 </Project>