Remove Layer 2 nav from dotnet-api-docs (#4197)

Removed the reference to the .Net L2 nav header from docfx.json for the dotnet-api-docs repo.

(The L2 has already been removed from the conceptual content for .NET in a previously merged PR)

Author: wadepickett

docfx.json

@@ -44,7 +44,7 @@
       "ms.author": "dotnetcontent",
       "ms.date": "04/30/2018",
       "searchScope": [".NET"],
-      "uhfHeaderId": "MSDocsHeader-DotNet",
+      "uhfHeaderId": [],
       "apiPlatform": "dotnet",
       "ms.topic": "managed-reference",
       "ms.prod": "dotnet-api",