Remove empty meta.source from ArtC example event (#284)

The backend.json examples for ArtC contained an empty meta.source
member. While technically okay it's uncalled for and can be confusing
for readers. It can also cause problems for tests of code that use
these examples as testdata.

examples/events/EiffelArtifactCreatedEvent/backend.json

@@ -3,8 +3,6 @@
     "type": "EiffelArtifactCreatedEvent",
     "version": "3.0.0",
     "time": 1234567890,
-    "source": {
-    },
     "id": "aaaaaaaa-bbbb-5ccc-8ddd-eeeeeeeeeee0"
   },
   "data": {