build(docs-infra): remove obsolete content from `example-config.json` in `setup` example (#36015)

The `unittesting` property in `example-config.json` is a remnant of the
setup (before `aio/` was moved into the `angular/angular` repo) and has
no effect any more.

PR Close #36015

aio/content/examples/setup/example-config.json

@@ -1,3 +0,0 @@
-{
-  "unittesting": true
-}