Remove empty source_url property (JuliaLang#445)

The track CI runs `configlet lint`, which currently exits with a
non-zero exit code if it sees an optional string key that has the value
of the empty string.

This commit therefore removes all such key/value pairs on the track -
they otherwise cause a failing CI check.

exercises/practice/robot-simulator/.meta/config.json

@@ -17,6 +17,5 @@
       ".meta/example.jl"
     ]
   },
-  "source": "Inspired by an interview question at a famous company.",
-  "source_url": ""
+  "source": "Inspired by an interview question at a famous company."
 }