Fix build dir to ensure we don't add targets for net5

Rx.NET/Source/src/System.Reactive/System.Reactive.csproj

@@ -172,14 +172,10 @@
 
   <ItemGroup>
     <None Include="build\_._" PackagePath="lib\netcoreapp3.1" Pack="true" />
-    <None Include="build\_._" PackagePath="buildTransitive\net5.0" Pack="true" />
+    <None Include="build\_._" PackagePath="build\net5.0" Pack="true" />
     <None Include="build\_._" PackagePath="buildTransitive\net5.0" Pack="true" />
     <None Include="build\System.Reactive.targets" PackagePath="buildTransitive\netcoreapp3.1" Pack="true" />
     <None Include="build\System.Reactive.targets" PackagePath="build\netcoreapp3.1" Pack="true" />
   </ItemGroup>
 
-  <ItemGroup>
-    <Service Include="{508349b6-6b84-4df5-91f0-309beebad82d}" />
-  </ItemGroup>
-
 </Project>