docs(cookbook/graphql): Remove specific .gitignore workaround thanks to angular#2803

Author: Urigo

public/docs/_examples/heroes-graphql/ts/.gitignore

@@ -7,5 +7,3 @@ dist
 !app/tsconfig.json
 !rollup-config.js
 !copy-dist-files.js
-
-!systemjs.config.extras.js