Remove duplicate yaml dependency.

[pq]

This duplicate entry is breaking pub get|upgrade:

Error on line 27, column 3 of /Users/me/.pub-cache/hosted/pub.dartlang.org/json_schema-1.0.3/pubspec.yaml: Duplicate mapping key.
  yaml: "^2.1.0"
  ^^^^

/cc @patefacio

[pq]

Thanks! 👍

I think to get things fully straightened out we need this too:

patefacio/id#2

If that's cool, it'd be fantastic to land it and publish new id and json_schema packages.