remove github package manager from nuget.config

(cherry picked from commit 9bee578)
(cherry picked from commit 30cd1dd)

Author: Mpdreamz

nuget.config

@@ -3,6 +3,5 @@
 	<packageSources>
 		<add key="Elastic Abstractions CI" value="https://ci.appveyor.com/nuget/elasticsearch-net-abstractions" />
 		<add key="Versioned Clients" value="https://ci.appveyor.com/nuget/elasticsearch-net" />
-		<add key="github" value="https://nuget.pkg.github.com/elastic/index.json" />
 	</packageSources>
 </configuration>