removed package references used when testing .netstandard 1.X

Author: Zyano

src/AngleSharp.Io/AngleSharp.Io.csproj

@@ -11,7 +11,6 @@
 
   <ItemGroup>
     <PackageReference Include="AngleSharp" Version="0.13.0" />
-    <PackageReference Include="System.Net.WebSockets.Client" Version="4.3.2" />
   </ItemGroup>
   <ItemGroup Condition="'$(TargetFramework)' == 'net46'">
     <Reference Include="System.Net" />