Remove non-functional Asset TargetVersion

I thought including TargetVersion="[14.0,15.0)" on the Newtonsoft.Json asset
would prevent it from being installed on Visual Studio 2017.
This doesn't seem to work, so removing it.

Author: jcansdale

src/GitHub.VisualStudio/source.extension.vsixmanifest

@@ -37,7 +37,7 @@
     <Asset Type="Microsoft.VisualStudio.VsPackage" d:Source="Project" d:ProjectName="GitHub.StartPage" Path="|GitHub.StartPage;PkgdefProjectOutputGroup|" />
     <Asset Type="Microsoft.VisualStudio.MefComponent" d:Source="Project" d:ProjectName="GitHub.InlineReviews" Path="|GitHub.InlineReviews|" />
     <Asset Type="Microsoft.VisualStudio.MefComponent" d:Source="File" Path="Rothko.dll" />
-    <Asset Type="Microsoft.VisualStudio.Assembly" d:Source="File" Path="Newtonsoft.Json.dll" TargetVersion="[14.0,15.0)" AssemblyName="Newtonsoft.Json, Version=9.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed" />
+    <Asset Type="Microsoft.VisualStudio.Assembly" d:Source="File" Path="Newtonsoft.Json.dll" AssemblyName="Newtonsoft.Json, Version=9.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed" />
   </Assets>
   <Prerequisites>
     <Prerequisite Id="Microsoft.VisualStudio.Component.CoreEditor" Version="[15.0.25824.0,)" DisplayName="Visual Studio core editor" />