Trying to fix #24, removing System.Runtime reference from NET45x target

Author: MichaCo

src/DnsClient/DnsClient.csproj

@@ -60,10 +60,4 @@
   <PropertyGroup Condition=" '$(TargetFramework)' == 'netstandard2.0' ">
     <DefineConstants>$(DefineConstants);PORTABLE</DefineConstants>
   </PropertyGroup>
-
-  <ItemGroup Condition=" '$(TargetFramework)' == 'net45' ">
-    <Reference Include="System.Runtime" />
-    <Reference Include="System" />
-    <Reference Include="Microsoft.CSharp" />
-  </ItemGroup>
 </Project>