rm System.Runtime reference

Author: eli-darkly

src/LaunchDarkly.EventSource/LaunchDarkly.EventSource.csproj

@@ -23,7 +23,6 @@
   <ItemGroup Condition=" '$(TargetFramework)' == 'net45' ">
     <Reference Include="System" />
     <Reference Include="Microsoft.CSharp" />
-    <Reference Include="System.Runtime" />
     <Reference Include="System.Net.Http" />
   </ItemGroup>