Remove redundant key/value pairs from config.json

The slug is not used anywhere. We initialize a track based on knowing the Track ID.
Since the repository is always named after the track ID, this field, too, is redundant,
as it can be inferred.

config.json

 {
-  "slug": "java",
   "language": "Java",
-  "repository": "https://github.com/exercism/java",
   "active": true,
   "exercises": [
     {