Remove unneeded System.Net.Http dependency

See https://github.com/dotnet/corefx/issues/19914#issuecomment-529113925

Author: ysangkok

build/JsonRpcSharp.nuspec

@@ -24,7 +24,6 @@
                 <dependency id="System.Net.Sockets" version="4.3.0" exclude="Build,Analyzers" />
                 <dependency id="System.IO.Pipelines" version="4.5.3" exclude="Build,Analyzers" />
                 <dependency id="System.IO.Pipes" version="4.3.0" exclude="Build,Analyzers" />
-                <dependency id="System.Net.Http" version="4.0.0" exclude="Build,Analyzers" />
                 <dependency id="System.Net.WebSockets.Client" version="4.3.2" exclude="Build,Analyzers" />
             </group>
             <group targetFramework="netstandard2.0">

src/JsonRpcSharp.RpcClient/JsonRpcSharp.RpcClient.csproj

@@ -13,6 +13,6 @@
     <ProjectReference Include="..\JsonRpcSharp.Client\JsonRpcSharp.Client.csproj" />
   </ItemGroup>
   <ItemGroup>
-    <PackageReference Include="System.Net.Http" Version="4.3.3" Condition="'$(TargetFramework)' == 'net46'"/>
+    <Reference Include="System.Net.Http" Version="4.3.3" />
   </ItemGroup>
 </Project>