Use implicit refs

Rx.NET/Source/tests/Tests.System.Reactive/Tests.System.Reactive.csproj

@@ -4,7 +4,7 @@
     <NoWarn>$(NoWarn);CS0618</NoWarn>
   </PropertyGroup>
 
-  <PropertyGroup Condition="'$(TargetFramework)' == 'netcoreapp3.0'">
+  <PropertyGroup Condition="'$(TargetFramework)' == 'netcoreapp3.0' or '$(TargetFramework)' == 'net46'">
     <UseWPF>true</UseWPF>
     <UseWindowsForms>true</UseWindowsForms>
   </PropertyGroup>
@@ -15,11 +15,7 @@
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </Content>
   </ItemGroup>
-  <ItemGroup Condition=" '$(TargetFramework)' == 'net45' or '$(TargetFramework)' == 'net46'">
-    <Reference Include="System.Windows" />
-    <Reference Include="System.Windows.Forms" />
-    <Reference Include="WindowsBase" />
-  </ItemGroup>
+
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.0.0-preview-20181205-02" />    
     <PackageReference Include="xunit" Version="2.4.1" />