Remove a dependency on System.Threading.Tasks.Extensions when targeting .NET Core 3 as it's not required

Ix.NET/Source/System.Linq.Async/System.Linq.Async.csproj

@@ -18,8 +18,10 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.5.2" />
-    <PackageReference Condition="'$(TargetFramework)' == 'netstandard1.4' " Include="System.Reflection.TypeExtensions" Version="4.5.1" />
+    <PackageReference Condition="'$(TargetFramework)' != 'netcoreapp3.0' "
+                      Include="System.Threading.Tasks.Extensions" Version="4.5.2" />
+    <PackageReference Condition="'$(TargetFramework)' == 'netstandard1.4' "
+                      Include="System.Reflection.TypeExtensions" Version="4.5.1" />
     <ReferenceAssemblyProjectReference Include="..\refs\System.Linq.Async.Ref\System.Linq.Async.Ref.csproj" />
   </ItemGroup>
 

Ix.NET/Source/refs/System.Linq.Async.Ref/System.Linq.Async.Ref.csproj

@@ -9,8 +9,10 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.5.2" />
-    <PackageReference Condition="'$(TargetFramework)' == 'netstandard1.4' " Include="System.Reflection.TypeExtensions" Version="4.5.1" />
+    <PackageReference Condition="'$(TargetFramework)' != 'netcoreapp3.0' "
+                      Include="System.Threading.Tasks.Extensions" Version="4.5.2" />
+    <PackageReference Condition="'$(TargetFramework)' == 'netstandard1.4' "
+                      Include="System.Reflection.TypeExtensions" Version="4.5.1" />
   </ItemGroup>
 
   <ItemGroup>