Delete ignored key from config.json

Since the exercise implementations are all in the exercises directory
we no longer need to ignore any non-exercise directories in the root
of the track.

config.json

     "strain",
     "trinary"
   ],
-  "ignored": [
-    "_template",
-    ".gradle",
-    "build",
-    "docs",
-    "gradle",
-    "scripts"
-  ],
   "foregone": [
     "leap",
     "grains",