Limit target platforms for integration tests.

Rx.NET/Integration/WindowsDesktopTests/WindowsDesktopTests.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>netcoreapp3.1;net5.0;net5.0-windows10.0.19041</TargetFrameworks>
+    <TargetFrameworks>net5.0;net5.0-windows10.0.19041</TargetFrameworks>
     <NoWarn>$(NoWarn);CS0618</NoWarn>
     <LangVersion>latest</LangVersion>
     <AssemblyName>Tests.System.Reactive</AssemblyName>
@@ -8,7 +8,6 @@
     <SignAssembly>true</SignAssembly>
     <AssemblyOriginatorKeyFile>..\..\Source\ReactiveX.snk</AssemblyOriginatorKeyFile>
   </PropertyGroup>
-  
 
   <PropertyGroup Condition="'$(TargetFramework)' == 'netcoreapp3.1'">
     <UseWPF >true</UseWPF>