Revert "Bump System.Reactive from 3.1.1 to 4.1.5 in /Rx.NET/Source"

Rx.NET/Source/benchmarks/Benchmarks.System.Reactive/Benchmarks.System.Reactive.csproj

@@ -26,11 +26,11 @@
   </ItemGroup>
 
   <ItemGroup Condition="'$(Configuration)|$(Platform)'=='Rx.net 3.1.1|AnyCPU'">
-    <PackageReference Include="System.Reactive" Version="4.1.5" />
+    <PackageReference Include="System.Reactive" Version="3.1.1" />
   </ItemGroup>
 
   <ItemGroup Condition="'$(Configuration)|$(Platform)'=='Rx.net 4.0|AnyCPU'">
-    <PackageReference Include="System.Reactive" Version="4.1.5" />
+    <PackageReference Include="System.Reactive" Version="4.0.0" />
   </ItemGroup>
 
   <ItemGroup>