Remove addtionalProperties in runners schema

This resulted into failure on EE pipeline, for more information follow
from
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7600#note_105310648
onwards.

spec/fixtures/api/schemas/job/runners.json

@@ -8,6 +8,5 @@
     "online": { "type": "boolean" },
     "available": { "type": "boolean" },
     "settings_path": { "type": "string" }
-  },
-  "additionalProperties": false
+  }
 }